Lox,
I don't know if this phrase is used in UK family law, but in the US there is the concept of

"FAILURE TO PROTECT".

Custody of a child can be taken from a parent if they FAIL TO PROTECT that child. If you continue to let your daughter be in an unsafe situation, you can be found guilty of Failure to Protect and your child can be placed somewhere else.

Children are born into the environment that adults create around them. Adults have choices and the power to move away, change situations. Infants and children don't have that power. Take control of this situation and make that child safe from the drug user!