Use enough icing and they'll forgive you anything. If the cake crumbles, just stick it together with more icing. The cake is to be looked at by children, not judged by cookery experts.
Have lots of finger food, nothing too messy, nothing that needs utensils.
To save mess and concerns about cutting a cake into equal sizes, make cupcakes and give them once iced cupcake (with a smartie or candle on top) each. Great for large groups, picnics and if you send them to school teachers will love you if you provide this rather than a large cake to be shared (Make sure you provide a couple of extra cakes just in case).
If you want jelly, make lots of individual jellies in disposable plastic cups – half full and add a dollop of ice-cream or custard – to save time serving restless children.
Don't over-organise a party. A couple of traditional games (pass the parcel, pin something on the something, etc) which may or may not get played, some lively music, a video if they get restless, a stash of sweets or other small prizes just in case.
If some guests don't want to join in and would prefer to sit quietly playing with the host's Lego, let them.
You'll need to clean up afterwards so don't waste your energy cleaning up too much before the party. No child ever went home and said “Gee that party was in such a clean house”
Provide plenty of drinks, including water.
Keep paper towels and a couple of large rubbish bags on hand.
Relax, it's just another birthday party and the kids are more interested in each other than your cooking, cleaning or entertaining skills. Invite a few of your friends, have a chat and let the kids get on with it.