In terms of fee structure for a children's party I would expect this to be on a per head basis, or depending on venue it may be a venue hire plus per head basis. Including adults in the mix might be more difficult, though it probably depends on what drinks etc you would be offering.
I would try to manage expectation though. You can throw a good party for children or you can throw a good party for adults, but trying to do both may prove challenging.
The best way IMO to have a good party for both children and adults is to have a big enough room or house for them not to be mixed all the time, and to hire a carefully chosen entertainer to keep the LOs happy. Not cheap, though.
Good things to look out for in an entertainer: ^Personal recommendation.^ A clown who does not wear makeup, or else makes up in front of the children. Magic involving small animals which the LOs get to stroke. Balloon modeling. Puppets.
Don't do party bags. They are a waste of money, the LOs don't need the sweets and the parents don't need the extra tat. Just give them one of the balloons and a piece of cake wrapped in a napkin as they leave.
It always felt wrong to pay someone else to run a party for me, until I realised that ds1's 5th birthday would fall on dc3's due date. So we hired an entertainer to make sure that the party could go ahead no matter what state I was in. It was fantastic. The easiest, most relaxing party we had ever had for the dc. Haven't had a single party at home since then .
Budget clearly is a big factor - but when you've not paid previously unless you'v ehad or know someone who has had a great experience. changing over from doing it yourself to letting someone else do it is difficult I think. Have been to some of my (adult)friends very lavish parties and am wondering if I can borrow a few of those ideas, without going all MTV Sweet 16 on my 2 year old - have you seen that show? Not a good add for parenting teens!!