Don't worry too much about he kit you're meant to have. Yes, legally, you need a hi-vis jacket for everyone, and a warning triangle, and a fire extinguisher. But the number of fines issued for this annually is pretty much zero.
There used to be (he may still be there) a big scary cardboard cutout gendarme inside the terminal at Folkestone with a list of things that you had to buy now (at 5x French shop prices) or risk deportation and having your car crushed, or something. The French police have an awful lot of better things to do than pick on UK holidaymakers.
Something you can maybe get the kids involved with is spotting speed camera (it's "le radar" in French) warning signs, and then the actual camera as you go past. The signs and the cameras are mostly pretty big and visible. The camera is generally about a kilometre after the sign, but there are sneaky stretches where they say "cameras for the next 15km" (even though there's only one).
At this time of the year, watch out for scams if you stop on the motorway. Someone may approach the car gesturing that your car (or perhaps theirs) has a problem. Either ignore them, or if you want to interact with them, assume that an accomplice may be looking to take advantage of your distraction to steal things from the car. So lock all the doors, and then just the driver gets out.