Fleet Air Museum is a v. long way from Bath! Agree that Victoria Park (if the weather is good) has a fabulous playground. There is usually parking available as well. Victoria Park is also adjacent to the Royal Crescent/Circus and there are some excellent child-friendly eating places nearby (Lime Lounge anyone?)
Other options would be the recently disabled-access-facilitated Roman Baths, which has lots for even v. young children to see, or the small but perfectly-formed and wonderful Holburne Museum (free entrance, child-friendly and also fully-accessible!) with a lovely cafe backing onto the Sydney Pleasure Gardens.
I also second Victoria Park. Good walking for old and young, play area has plenty of benches. Take a picnic or wander along to the Marlborough Tavern for a yummy lunch [their roasts are delicious and generous]
Avon Valley is also good, haven't been with DD, but have taken Rainbows, Brownies and Guides on day's out there over the years and it's always proved a hit.
@Bristol is amazingly fun for all ages, the littles will love it and there is good flat walking about for after. The weather looks good so it may be worth taking a change of clothes for the littles as there is a fountain in millenium square which DD 18 mo and Dnephew 3 loved splashing for about an hour afterwards. A walk away is the MShed which is the free Bristol museum on the local heritage, quite good for local interest. You've also got the SS Great Britain along again, which is alright, it has lifts to get you between levels and down into the dry docks, but the floors are quite uneven. I managed to trip over most things
We are bringing the DCs (5 and 2.5) down to Bath area next weekend to meet up with overseas relatives. A 70 year old woman who walks with a walker may be involved, though she would sit on a bench happily during any energetic bits. Any ideas on what might suit all of us? So far I am wondering about the Fleet Air Museum or the Midsomer Norton railway? Happy to come into central Bath but not sure if there is much for DCs. Don't mind high entrance charges or whatever as haven't seen the relatIves for years!