A fun day out with your little ones can be enormously rewarding all round; have you ever considered an outing to watch aeroplanes? You might think that airports are only for international commuters and holidaymakers, but they are often easily accessible to anyone to simply turn up and watch to explore what is going on.
Parking costs at airports can be prohibitively expensive, but they are generally well served by public transport, with buses and trains bringing people in. If getting to an airport by public transport is too complex from where you live, try driving to a nearby location where you can pick up a bus or train for the last part. Many airports have special recreational visitor centres, some with family activities running regularly.
If you don't live close to an airport, then do some research to see if there are any airfields nearby that allow you to enter and watch light aircraft coming and going. Airfields won't be serviced by the same public transport network that airports benefit from, but there is quite likely parking available on site and it may also be free. Airfields often have a public terminal which you may be able to access and may well have a nice coffee shop for you to sit in, overlooking the airfield.
The highlight of an airport visit will be watching planes take off and land, but airports offer plenty more to observe too. Watch planes being serviced - loaded with bags, meals and fuel; look at the check-in desks with bags flying everywhere; and watch the crowds of people coming and going. Use the day as a point of conversation to talk about air travel and worldwide destinations and undertake related activities on your return.
Do remember that airports and airfields are sensitive about national security, so don't park or loiter in places that you shouldn't be or you may well find you are moved on by authorities or the police.