Another good and cheap site is https://www.lets-have-a-party.co.uk
Just a few ideas from the top of my head that should hold for kids as well as adults. Basic rules are: accessories make the costume, it should be nigh on impossible to move, drink, eat or be recognised in, you should look like an idiot, and you should never take the costume off until the night is done.
For over 45 shows in January and February
There is one week until the deadline
It should help you save £667.95