I stumbled across this site, and I love it.
It's easy to navigate, fast loading, has great graphics and the games are fun.
And it's by the BBC, so you know your kids will also learn something.
There are games focussing on spelling, phonics, rhyming words, alphabetical order, synonyms, punctuation, and sentence construction. The site also has math games.
The site is called KS1 Bitesize.
If your kids try it, please let me know how they liked it, and whether you think it will help their literacy skills.