If line 30 is blank you are ok. You don't need to enter the full 4200you got. You only qualify for 2800 and you already got that (actually you got more).
What do you mean you could have claimed her? That may change everything. Even if she files a return you can still claim her if she qualifies as your dependent. And she has to say on her return she can be claimed even if you don't claim her.
