Been to both and love both. Hever has a really good water maze (but my kids are older), and a hedge maze, also a play area (but probably too old for a nearly 2 year old). Grounds are lovely, but never get to see much of the castle as my two have the attention span of goldfish for things like that. Leeds grounds are bigger (and their ticket will last you all year), got an avery, a huge maze and a nice toddler play area. I'd probably plump for Leeds with a wee one. They also have a land train, so when knackered on the return you don't have to walk the whole way!