I am sorry to read this. Of course you worry for your kids if you have reasons to believe their dad and friends may be drinking, etc, while they sleep over with him.
I can picture you without the kids, pondering and worrying that they may not be safe. That must be awful for you if you’re sitting alone on a Saturday night, for example.
It is unfortunate your friends or family don’t appreciate your dilemma. It makes your place more isolated and lonely. You do not say if you are in a relationship, but if you’re not, your path with this is a more lonely one.
Can you speak to your husband in a mature, appropriate manner, to share your concerns? Perhaps pre-empt the conversation by letting him know you believe you are both singing from the same hymn sheet. That this is about the kids and you are not criticising him and wish him well in his new relationship. Do you think this may calm the waters?
It is better, in my opinion and experience, if parents communicate without the need of a legal facilitator. Keep it open and respectful.
If you cannot speak to your ex, is there a mutual friend or family member of his whom you could voice your concern to? If this is not possible, unfortunately you will have little choice but to consult your solicitor.
Another option may be to ask your ex if her would be willing to attend a counsellor/family therapist to discuss this with you.