Day out half-way between Fleet and Cheltenham - or Oxford?

[DontCallMeBaby]

I'm trying to organise a meet-up (on a Monday in the school hols, if that makes a difference) with my cousin and her two DDs. I'm in Cheltenham, she's in Fleet, so looking for something about halfway.

An alternative to properly half-way might be Oxford, which is a bit over an hour from both of us. Any ideas, anyone, either in Oxford or anywhere else roughly equidistant between us, and not much over an hour away?

DDs are 5, 7 and 9.

Millets farm centre near Oxford. Has animals to look at, small play area, a maze, cafe, shop.
Blenheim palace. Lovely gardens, maze, play area. Take a picnic.
If you go to Oxford, use one of the park and rides. And make sure you go to one of the G&D cafes for ice cream.

Bekonscot Model Village in Beaconsfield.

Ooh, I didn't know Beaconsfield was there ... my geography is so rubbish.

Millets farm actually looks like a really good one - almost exactly equidistant and only just over an hour. And has a yummy looking cake on the homepage.

Thanks!