You can't sack a mother.

You can't employ a new one.

Even if you could or wanted to eclipse the mother from the childs life completely, it is not a decision that a father has the right to make, legally or morally.

The child won't really miss their childminder or be traumatised by their sacking like they will their mother.

The child won't need sensitive and considerate understanding throughout their upbringing on the subject of their childminder as regards their questions, understanding and experience of their mother and their relationship with her and her fathers relationship with her and (as she gets older) her mothers behaviour and addiction etc.

The two scenario's are completely incomparable except in the respect that they are doing drugs on the one hand, and expected to give care on the other.

A childs welfare and emotional health run much deeper than that.

PS, who is this lovely sounding mother? I'd love to meet her. Her partner and kids are very lucky to have her.