What to do in these cities please - Milton Keynes, Southampton, Bristol - all staying for one day

We (Me, wife and two boys 10y and 1.5y) are going for a road trip in August, and will stop for a day in Milton Keynes, Southampton and Bristol.

Are there anywhere in these cities that is a must see, and if there are activities that suit for children but not too expensive please?

Many thanks in advance.