Bluetits attacked

Hello

we have a bird box with a camera in it and for the past 7 yrs we have had Bluetit families in there.

last night when we were watching it with our 4 yr old daughter, mummy bird went out and another Bluetit came in at first we thought it may of been daddy bird but it started pushing  the week old chicks out of the nest, we thought it was strange and wasn't sure what was happening, we turned it off as we didn't want to upset our 4 year old. We put it back on 10 minuets latter and the other bird had gone and mummy bird was sat on the chicks keeping them warm we could hear the chirps.

this morning our daughter put the camera on like she does everyday and she started screaming they had all been attacked and were dead

is this normal behaviour for blue tits to attack bluetits?