Are you going to fly for fun, or to get somewhere?
Flying for fun: Don't attempt it with kids younger than about six to eight years old. They don't appreciate the novelty/excitement factor of it and will just fall asleep out of boredom. It's a waste of money.
In any case: If the kids are old enough to sit upright (2+ I guess), use enough cushions and booster seats so that they can see out. If you want to let them sit upfront, remember that a typical GA aircraft cowling is a lot higher than a cars instrument panel, so you need a lot of additional height for them to look over the cowling. I have had to use full booster seats (cushions were not enough) to let kids up to 12 years of age look over the cowling - something they grew out of years ago in their parents car.
Make sure that they're seated with a seatbelt securely and safely fastened. This may help:
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Get children's ear defenders or even child headsets. These are 30% smaller than adult headsets. I like the SkyLite headsets. Skycap aviation in the Netherlands is the distributor.
https://www.skycap.nl/ Also think of decent sunglasses.
For babies and toddlers, equalizing ears is not something you can teach or tell them, so a problem. Plan your descents very, very gradually. If possible less than 300 ft/min, possibly with level steps in between. Having something to suck on may help, but don't give chewing gum or sweets that could be swallowed and block their windpipe. A pacifier may work best.
Have sickbags, babywipes and such handy for the results of turbulence.
Keep the initial flights short (<30 minutes) until both you and the kids are more used to the situation.
Have an adult on board to take care of the kids, both in the aircraft and on the ground. You've got enough to do already without their distraction.
If you consider letting the kids have a go at the controls, do it only once the aircraft is in cruise flight and properly trimmed. Use the native language of the kid, not "aviation English". Agree beforehand on how you hand over controls. And for a lot of kids, if you say "My controls" they don't have a positive command as to where to put their hands. If you say "Now put your hands on your knees again" instead, there is a positive command on where the hands need to go. Works much better.
Don't expect they'll be able to read instruments so if you want them to navigate, use outside landmarks where to fly to. And verify that the landmark is visible to them - despite the booster seats and everything they may not be able to see a landmark if it's on your side. For the same reason, letting them keep proper altitude is even harder - they simply won't understand the instrument, let alone what to do with the information.