Play safe and get it at home, especially if it's buy one get one free.
You can get sun cream in Goa but the brands which you will recognise, Nivea etc. aren't that much cheaper. The local concoctions might fry you alive!!
We've never burned with the Boots Soltan range and you still get a good tan.
If you have a Home Bargains where you live, they are currently selling Piz Buin at £3.99, BARGAIN

Sun Creams... You can buy them over here, you can pop in and visit us and get some off us... We have lots of makes, we only expect a small donation which would be less than you'd pay in England or in Goa. Tourists give it us but obviously the children don't use it.

Or just by it and leave it with us when you leave....

Boots the Chemist are doing buy one get one free just now on their Soltan makes. Ive used them before theyre really good.
Rob's idea is a very good one as well and for a great cause!

for advice on buying suntan lotion etc here we have 22 pages to read here!