'we understand that you would probably like to buy (child's name) a birthday present but he/she really has more than enough toys, if you feel you would like to buy a gift however, may we suggest a book or a voucher for an activity?' Thankyou. ?????
It's a bit long winded I suppose, but it gets the point across.
" this year we are not asking for any presents, however if you feel strongly that you want to buy something, please feel free to donate some money to the 'keep mum in alcohol fund' you can donate by cash, cheque and all major credit cards... sort code and account number on request"
I usually ask what children would like if mine are the invitees.
If we are having the party I am just glad that people turn up, and have realised that I just have to put up with the tat presents.
IMO if the parents don't ask what you specifically want then don't give specifications for presents - if any are like me they will have lots of tat presents in the cupboard bought en masse in the sales ready to give out and an instruction for something different would really annoy me!
Ds2 recently went to a party and the invite said something along the lines of .. instead of a present please donate to the NSPCC. It had an account name and I think the partygiver gets told afterwards how much they raised.