What about child size concrete mushroom painted in whimsical colors large enough for them to sit on? Also look into fairy houses. Maybe create a path with river rocks mixed with a few glass rocks. If the area gets sunshine you can use décor items that glow in the dark. This sounds like a fun project.

Using weaelemu's idea, why not add items like mushrooms etc. to make the children feel like they have been shrunk and in a fantasy world?

Large mushrooms and flowers
Water falls and mini streams
A mirror framed by an ivy of some type(something unexpected)
A miniature house (Hansel and Gretel?)
A giant chair(make them feel small)
A mad hatters tea table

there are many whimsical and fantastical ideas that you could use. just imagine you are a child again and have fun.

Good luck. Be sure to upload some photos!