advice needed

[lou85]

Hiya im looking for some advice...... yesterday my children went off to there dads and his girlfriends for the night he has them on the first and third sat of the month.

This morning i get an urgent phonecall from him saying to come and pick the kids up as they were having an argument and the girlfriend had hit him.
The kids did not see this but heard it all very clearly there is obvisiouly violence going on and i do not want the children to be part of it on top of it all i find out he went out all night and left the kids with her without me knowing.
I feel really strongly the children should not be put through this while visiting there dad.
does anyone know where i stand legally and if i can just stop the kids going their.