pregnant and eating other babies

Question
My son bought 4 white mice.  We realised at least 1 was pregant and 3 were female.
The day after 1 gave birth and the male was trying to mate again.  We removed him and gave him a new cage, he is played with lots by our 6 yeard old so is happy.
The babies are 1 week 2 days and mum moved them into the house in her cage, but another pregnant mouse had started nesting n there.
They were happy until last night when I heard lots of squeaking.  I went t investigate to find the pregnant one had eaten 1 and was starting on another.
We moved dad into a large plastic safe and secure tub and put the pregnant one in the cage.  We left mum and babies in her own cage to limit the stress.  She finished eating the baby which I thought she might.
The other one looks like she maybe going into labour, could this be the reason she attacked the babies?
I kept mice when I was younger and know that sometimes mums eat their young for reasons but have never heard of others eating them.
I am also concerned that I may have caused stress to the pregnant one by moving her, she seems to be panting a lot and been very restless.
Thank you

Answer
Your pregnant mouse is doing this to make room for her litter. She is ready to give birth any day now.
Even though mice will live together sometimes there is cannibalism if there is more than 1 litter in the colony and if it is close set, i have also seen this with gerbils, so it is a good idea to separate any suspect pregnant mice from the colony.
As soon as they are weaned you must split the sexes (around 4 weeks) or the boys will mate with mum and their sisters.