I have moved overseas and have many ex-pat friends who have moved overseas and I don't at all advise giving anything that has to fit in luggage.
How about a season pass to some educational attraction in London? I LOVE the Tate Gallery, but it is not so much for kids.
Maybe there is some kind of kindermusic style class that you can investigate online and buy her a gift cert. for that. You can e-mail it to her and she doesn't even have to print it out until she is ready to use it.
No matter how small your gift, every inch of luggage space is precious (especially with two small folks in tow!) and whatever you give her will likely be left behind.
Alternatively, you can present her with the gift and let her know that she can't take it home yet because you want to mail it to her, thereby saving her the luggage space. She'll appreciate that!
You're a good friend.