The unicorn theme is such a pretty and magical theme to play with, see what you have around the home first too! My daughter had some large unicorn teddies I could use to decorate and use as play things for the party.
Hire a bouncy castle, you can get castles to suit most budgets just check the company has insurance and follows safety procedures. Having a bouncy castle create a space for the kids to get together, bounce and let of steam and energy. Its a must at any of my parties!
FREE or CHEAP Party Idea:
Set up an activity table, this could be colouring, DIY nail bar, temporary tattoos, face gems or a craft. This is a calming activity and the kids will love doing each other nails or tattoos. Colouring is also calming activity and gives the kids a time and place to chill out for a moment. In all my experiences of parties having such an area works so well and creates happier party children.
I find a craft or colouring activity works for the adults too, its something the adults can do whilst attending the party. And it also gives you 5mins to stop, slow down and enjoy the party too.
If you have the budget you could hire a face painter!
Bubbles are so magical, even if you just have packs of small bubbles for the kids to blow. Or get a bubble machine! Or hire a bubble entertainer if you have the budget. Or get yourself a kid inside a bubble hoop.
If your budget stretches then book a unicorn pony visit like this one, its such a magical and exciting thing to look forward to and share with all your class mates. My daughter will not be forgetting this in a hurry.
If you live in the Gloucestershire or Bristol areas you could book this exact party from me see my party entertainment page for more party entertainer options.