You're a contractor, and you haven't noticed the earning potential here?
What you need to do is delegate the cost. Now, to get the most out of the price, you need to have 30 children at the party. When the parents arrive, dismiss the parents, thus leaving you, DW and whoever in charge. (You may need to hire two more parents for this.
Now, unless the parents are hired help, you charge £5.00 entry, for four hours.
Then, childcare. Again, hired help get this free of charge. Now, as it's daytime, charge £3.25 per hour, per child.
Add to this, the cost of insurance. If little blighter gets hurt, you refund the parent a fiver, give little mite a magic sticker and lollipop. Insurance should cost £2 per head, covering the whole session
Now, add to this the cost of food. Unless the parents supply a packed lunch, £3 sounds reasonable for a buffet.
Entry (27 x 5 = £135)
Childcare (27 x 13 = £351)
Insurance (27 x 2 = £54)
Food (27 x 3 = £81)
Total = £23 per child.
Total income = £621.
Now. Obviously, you'll have to subtract wages. Each member of staff should be given a compulsorary, non paid hour long break. You should have no more than two members of staff. So, at minimum wage, you have two employees earning £17.79 each.
So; Wages (£17.79 x 2 = £35.58)
Meaning, altogether, income from the party so far comes to £585.42 - £440 = £145.42.
Your wife is a genius!