Wouldn't they learn both there to be fair? It doesn't really matter which primary language they learn in my book - children can make the most remarkable adaptations when young. The little 4 year old next door to my folks is tri-lingual in German, English and French, switching between all three flawlessly and without effort.
Secondary school's different mind, but I wouldn't be unduly concerned if my children learnt Welsh as their primary language at junior school, as long as they weren't prohibited from speaking English. You've got home time to teach them a decent language after all...