open top bus tour was interesting, fashion museum and the house on royal crescent.We didn't pay for the roman baths but looked at them from the pump room and had a cream tea. Theres a lovely park and some boat rides that may appeal to your ds.
I love Bath passionately! However, personally the Roman baths bored me but actually would appeal to my 9 year old son as they have recently studied the Romans and he gets way more excited by old rocks and archaeology than I do. So still prob a good place to go, queue was absolutely fine when we went although it was oct. the best bit of the baths, the main bath and not the old rocks below you can see from outside (sort of!) if it was just you and your husband I'd highly recommend the rooftop pool at the thermae bath spa fantastic experience. I'm sure your children would also love it but it is v expensive so I guess depends on budget. Wandering round the town and looking at the quirky shops is also great fun but again prob not with kids?! Sorry not much help!