Can the social servies take my children off me if I start a relashionship back up with a ex who was one violent but now getting help?

Possibly.
Only you and they know the extent of this story. If you went through Social Services to keep him away, then I would move cautiously regarding any involvement with him, especially when the children are around. Let him apply to court with proof of his program and request for visitation, and you can respond.

I didn't go through social services but the police contacted them after one asault my children were not present though.

Yes children protective service can and will take children out of a home if they believe the children are at risk or in danger.

Often only after the children are taken, do you have the opportunity to hire an attorney and file to try and prove they were wrong.

Also sorry, if not because of the kids, why do you want back with a ex who was violent with you. Please seek counseling for yourself to find a path to move on to a healthy relationshiip