Don't worry about entertaining them, Play some music and they will jump about for ages. Play some traditional party games. Keep them simple and they will all know what to do.
Try and rope in a more confident freind or family member to play games and DJ. My BIL was great and had more fun than the kids. I did the games along with a freind to help referee.
You could hire a small castle form £40-50 which will help take the presure off. We hired a hall at sports centre for £40 which included a castle. Great value.
Keep the food basic sandwiches, crisps, juice and ice cream. Anything more is a waste and just ends up on the floor. I added sweets, sauce and sprinkles for the kids to help themselves to for an Ice cream factory type thing which made the eating part of the party last longer. Less entertaining .
Once a slice of cake is in the bag you don't need much else try to aim for a £1 and that will save you £30.