You should try Premier Inn County Hall. Very child friendly. Book 2 adult breakfasts and children eat free, or something like that. Good size rooms to share.
Excellent location beside London Eye, 5 mins House of Parliament, big ben, 15 mins walk to 10 Downing street. Two tube lines 5 mins walk. I brought my grandson and we had the most wonderful time there. He loved the big breakfast, he was 13 at the time. Browtal