Ah! They are free however, if you get a family railcard, it will actually be cheaper for you and partner or other travelling adults. Absolutely ridiculous, but the way it works is you pay a one off fee for a travel card for a year, and then it enables you to have child's ticket for 1/2 price (if child is over 5 - negligible if younger) and (I think) up to 4 adults with 1/3 off each of their tickets. So it is a really worthwhile saving. Sorry if this isn't very clear - check with National rail service 08457 48 49 50.
The other thing to think about is how busy the train is going to be/whether you need to book seats etc - if you don't have a railcard and/or aren't buying a seat for the child then you can't book them a seat. Most journeys of course, that doesn't matter - as long as you can reckon on there being plenty seats.