@MustIgo it’s a really tough situation and you’re doing the right thing by questioning and keeping an eye on it. Keep checking in with your DC, keep making sure they are happy with arrangements (and with themselves). Try not to make a situation where they couldn’t have the option not to go if that makes sense?
If my DC ever say they don’t want to visit someone I won’t make them, and if I can’t leave one or both with my DH at the time I was intending to take them with me, then I won’t go, and I will rearrange for another time. I never want to put my DC in a situation where they have to go and spend time with someone they don’t want to see. And I definitely wouldn’t leave them alone with someone if they said that.
It might make life harder, but someone has to advocate for your DC, and they will remember when they grow up if you don’t. Obviously at the moment, your DC are saying they are happy to go, but I mean if that were to change.
One of the things I have learnt from the workshops I have done recently is not to try and “teach” your child what you think is wrong with someone in their life (like a bad influence). Try to make a kind of understanding with your child of what makes a good friend or a good relationship - make sure you both agree to the definition you come up with. A good friend does this, a good friend doesn’t do that etc. Then, when they talk about something someone has said or done, don’t be the one to rush in and say “they’re a bad friend, they’re not acting in your interests”. Try to let them be the one to draw that conclusion.
When they see it for themselves instead of being told, it really is their own opinion. You could say “Oh, so do you think they said that/did that for your benefit? No? Oh, why do you think they said that then?” etc. if they said yes you could question further “How do you think they thought saying that would help then?” or something. It might even help to do this with your husband?