Lets look at modern Who and the children, this is from memory so please bare with me.
1. The empty Child - yeah that kid ruined that episode
2. School Reunion - all those kids, pah
3. Fear Her - (ok, she was a bit annoying)
4. The family of Blood - that girl with that balloon, totally ruined it
5. The 11th Hour - that young scottish one, dear oh dear
6. The big Bang - oh my lord, it's that scottish one again!
7. The Almost people - that little boy did an excited dance - just what gives huh? I turned off
8. Night Terrors - the dolls house I could handle, but the boy!! Sheesh
9. The Doctor, the widow and the wardrobe - more kids ruining the show
I am sure I have missed one or two, but I hope you get my point. With the exception of fear her all the children have added something to the show.