If you have two children, I think you find you do a lot with the first child and then forget to do them again for the second child. Or at least do them again when your second child can remember it.
At least that was what happened with us and Stonehenge. We realised that Kieran hadn't been there so we set out to correct that with a little field trip since it isn't that far away. And we went to Averbury too since that fits in well with the theme of ancient circles. The museum at Avebury is very good and there's a decent cafe for nice coffee.
It's an excellent trip but choose your day carefully. Stonehenge on an overcast day is a bit dull. You need sun or some dramatic cloud formations to bring pictures alive. There's inevitably a cold wind blowing over the Salisbury plain so be prepared for that. A car picnic is a good idea.