I want to claim for free milk but the phone number i have says it's better to write to them. what should i write? will they send me a form or do i have to state all about my kids in the initial letter?

The claim form asks for details of numbers of children so no need to say that.

You just need to ask to register and give them your details.

Don't forget, you need to put any money you get back as income through your books. :D