I grew up riding on open deck cotton pickers-- heck I had the job of crawling under the thing to clean the back doors out and pulling tie vines off the snouts in weedy fields. With cabs on tractors nowdays it's a lot cleaner and safer.
If you're going to take a kiddo along, make sure that safety is absolutely job one. I wouldn't even consider taking a kid that little on an open-station tractor-- yeah, you might have a seat bolted down, but bolts can work loose or break or other unforeseen stuff, and there's no "backup" like there is inside a cab. Too much dust, heat, and noise on an open deck as well IMHO. Earplugs cut down the noise (which should already be safely tolerable inside a cab) but they don't eliminate it and can be uncomfortable and troublesome to keep in kid's ears, plus they're more vulnerable to hearing damage at younger ages (IIRC). I'd say unless you're running a cab tractor, forget it til their older. Even then, I would only allow them to ride doing certain jobs (not shredding or mowing or disking or baling-- anything that could result in flying debris or requires your undivided attention).
Even in a cab, they need to be "belted in" to some kind of buddy seat or whatever. From a rollover standpoint, the driver is technically supposed to be seatbelted in to prevent being ejected in a tractor rollover. Those windows are the first thing to go, and anything tumbling around loose in there (including the driver and/or kids) can fall out the window hole and be crushed as the tractor rolls over). Yeah, I know it's probably a very remote possibility, BUT, it CAN and DOES happen... better safe than sorry! Again, for certain jobs, like mowing or baling that basically requires your undivided attention (and mowing can result in thrown objects from disk mower bars possibly shattering the glass and flying through the cab-- again rare but NOT unheard of!) I'd leave the kiddos at home.
The other issue is, when you inevitably have to stop and get out of the tractor to fix something, make an adjustment, etc, BE AWARE that leaving a kiddo in the cab is a RISK! Kid's love to "imitate dad" and may start moving the levers or pushing buttons at just the wrong time, with possibly catastrophic and tragic results. I know even as a bus driver I had that happen one time-- we were required to "walk kids across the road" on main highways for kids that lived on the left side of the road from the buses' direction of travel, and one morning while walking across in front of the bus to get the kid from the mother who'd escorted her to the middle of the road, I heard my air brakes being "fanned" (someone stomped the brake pedal). I rapidly glanced over my shoulder into the windshield and saw a kid dart back into his seat, so know who did it. Despite the bus being in neutral (bus automatic transmissions don't have a "park" slot) and the spring brakes set (park knob pulled out) I read the kid the riot act, because we were parked on a slight incline back down to the bridge over the creek, occasionally with other cars pulled up behind the stopped bus. I told the kid that if he moved the wrong lever or pushed the wrong button, he could have either run over us or turned the bus loose to roll back and crash into cars behind it, or roll back down the incline and off the bridge into the creek. OF course I wrote the kid up when I got to school and he was kicked off the bus for the rest of the year. Fortunately nothing bad happened. Others haven't been so lucky-- a friend of my Grandpa used to get out of his pickup with it idling to open the gate into his pasture, and one time the dog knocked the thing into gear-- the truck ran him down and hung up on top of him, and cooked him to death under the muffler. If a kid left in a cab were to pull the powershift lever or pop the tractor in gear, or raise or lower the hydraulics or shift the PTO in gear while you had your hands in the machine or were up under it, it would be a VERY bad day! I wouldn't consider getting out of a tractor with a kid left in the cab without turning the engine off and having the key in my hand-- just not worth the risk.
I'm all for kids doing stuff on the farm and being involved, but we have to make sure that THEY stay safe and don't get hurt, and that they can't inadvertantly hurt us or anything else. (and it's not just kids-- my BIL's Dad, who just turned 90, nearly ran me over with a gravity box while I was reaching up inside the open door, scraping down piles of wet corn from the corners with a 2x4... He climbed in the cab to pull the tractor up to unload the rear wagon, WHILE I WAS STILL NOT FINISHED WITH THE FIRST ONE! I instantly jumped rearward out of the gravity box door when I felt the powershift pop into gear, and started hollering at him, but he had driven over the corner of the swing-away auger and nearly upended the swing-away and cleaner before he realized he had screwed up! Gotta watch some of these old timers just as close as little kids-- they really want to help and be involved, but there comes a time when they just don't think as clearly or appreciate the dangers like they should-- one day I was running corn screenings from the cleaner up to the barn alley to spread it out to dry on the floor, and when I started back to the cleaner, I saw Grandpa fiddling about with a roller chain driving the auger and cleaner screen, turning at full speed! I held my breath hoping he wouldn't get his hand pulled into the chain before I got back down to the bin!) There's a LOT to be said about "age appropriate" activities!!!
Just remember to THINK SAFETY and "don't rule anything out"-- even stuff that seems highly improbable or unlikely, usually have consequences too terrible to contemplate. I've seen, heard, and read about too many accidents that were, tragically, for the most part, preventable. I read about a guy combining somewhere a year or two ago who had the kiddo in the cab with him-- the kid liked to lean up against the sloped combine cab front glass and look down into the header, and the glass popped out or broke, and the child fell into the header, and was killed before he could get the machine stopped. Another friend of my Dad's knew a neighbor whose little boy fell off the tractor while riding with him and went under the Bush Hog before he could get it stopped and was of course killed. Just don't take ANYTHING for granted!!!!
Later and good luck! OL J R