I have always encouraged regular contact (every other weekend) with my dc and their dad. However he's always been terrible with money and somehow is living in a 2 bed flat with his gf and her 3 dc (one of whom is their baby together). He admitted there was no room for our 3 dc and suggested they stay at his mother's house, but he would have them all weekend and drop them at his mum's at night.
I then discovered that although ex mil had a 3 bedroom house, she had her own double bedroom, her partner had his own double bedroom and my 3 dc, aged 9, 11 and 13 were expected to share bunk beds in a tiny box room. Ex had also been smoking in this box room and the whole room stank of stale fags. Obviously I then said that this was not suitable and until ex could provide suitable accommodation he could no longer have them overnight.
He continued to have the dc during the day on his weekends and they would come home to sleep. However 11 year old dd recently sent me a video of her dad's flat and I really don't want them to go there at all. There are black bags filled with rubbish on the floor. Random rubbish like empty milk bottles and coke cans lying around. A thick layer of dust and grime everywhere. Pet turtles living in their bath. The toilet was unspeakable. Brown. Looked like it had never been clean. I wanted to be sick.
I don't think they should spend any time there at all.