Help!

My ex and I have not been together since I told him I was pregnant, he ended our r'ship as he was not ready to be a father. When she was born he contacted me and advised he wanted to get to know his dd, so he has had a r'ship with her since.

Fast forward to now, our dd is 3.5 & he sees her once a week (thats all he wants) Four weeks ago while I was giving my daughter a bath, I asked her how her day was with daddy, she told me she was playing with these kids & they touched her, i asked her 'where baby' she pointed to her bits, i then asked 'where was daddy' she proceeded with 'downstairs mummy' at which I just redirected the conversation to something else. I was so upset, i thought why was he not supervising these kids.

Once I put my dd to bed, i contacted her father and asked a few questions to nut out if my dd was infact telling me the truth. Everything I asked my ex, ie if he was at his mates house who has kids, & the kids were upstairs, he confirmed, to which i then proceeded to tell him what our dd had said. He was shocked, he never thought this could happen. I advised him that kids are very curious and I dont want our dd to be a target so moving forward he is to supervise her while kids play where has clear view of their movements, to prevent this from escalating further. He agreed.

A week later my dd came back from her dads to tell me she was with those kids again, upstairs & daddy was downstairs with the older people. I was furious. He's also requested sleepovers now & I have no idea what to do.

I dont think I trust him anymore. I really need advice on this as I have no idea what to do! TIA