Surpise party for a 10 year old?

Okay that can be a bit difficult when they are that young, I do believe there exist companies that can help you with planning it, but they cost a fortune, so you're better off on your own, unless you got lots of excess money. So lets see:

Themes: What does she like? It should be personal, something she is interested in; Harry Potter? Princess party? Carnival?

To keep that a secret from her will probably be the most difficult tast, but what I think you should do is to have the party on a saturday, don't tell her friends, but their parents, that way the parents can make sure the kids don't got plans and are ready, but they won't have to tell them until that friday/saturday, just keep them away from your daughter, and you shouldn't have a problem!

To keep it hidden from the gues of honor could also be difficult, but I think it can be arranged by having the party in another location than your house, then there won't be a danger of her finding out, when it's time for the party just tell her you're going to visit someone, and there's your surprise waiting for you! (bring clothes for her to change into when she gets there!)

Fool her by saying "We can't afford a party this year" and tell the parents of the kids so they will keep the secret. She be so surprised.

just tel the perents then they can tell their kids the day of! and as for surprising her tell her to take a ride with you to get baloons.

theme- maybe a spa party. they are always fun! or like a dress up party!