There is something to be said, good and bad, about all 3.
Cons
Wild Animal Park-very hot this time of year. Alot of walking-very steep in places.
Zoo-also can be very hot in the summer and alot of the animals hide during the heat. You can do the Zoo at night but you really won't see many animals.
Legoland.-Also very hot (do you see a theme here?) Not alot for smaller children to do.
Pros-
WAP-animals are in more natural setting.
Zoo-bus
Legoland-water play area
Where would I go? You're already doing my first choice-SW. We have AP to WAP and ZOO and we've been to Legoland twice. I think I would do.........Legoland. If you want to see animals, at this time of year, I guess I would do the zoo. WAP is too hot. But, wherever you go, plan on getting there early. If it gets too hot you could leave and come back in the evening.
Even though we have AP we don't go during the summer to WAP or the Zoo-it's just too hot!
Personal info:
Me, mom 47
DD

, 6
and usually my DM, 66