I would keep the kids at yours for now and speak to her and try and get things sorted, you really wouldn’t want to send them back and for something to go really tits up and your regret that forever.
Im not talking about the drugs necessarily just sending them into a situation where she isnt coping. I Would try my hardest to keep social services out of it if possible (1st hand experience tells me they will make things difficult for all of you for a long time if they get involved).
If you say you are having them and she needs time to sort herself out then I cant see that being unreasonable.
Good luck