Hi,
I was searching Amazon looking at different bipolar books and found two of them geared for children to explain Bipolar...you might look into those.

In the mean time, just explain that Mommy has her ups and downs and sometimes she's really sad, other times she's really happy and that it's a sickness like the flu that you see a doctor to help fix is my best advice...The 11 year old might even be able to understand further, but the former suggestion is really geared toward the seven year old.