Legal help and advice please?

Ok my ex has dated his g/f for 2 months then moved her in, she has 2 kids that live with their dad and she gets them every other weekend she has a 13 yr old daughter and a 11yr old boy who share a room with my 9 yr and 6 yr old boys. they live in a 2 bedroom apt. My 9 yr old hates the new g/f and the last time my boys were with him they went to a family event for her side and my 6 yr old was choked and kicked in the ribs. when i picked them up i had to take him to the er and they diagnosed him with bruised ribs. 8 months ago he had a different g/f living with him claiming he was going to marry her to but ended up kicking her out. What can i do? can i have his overnights taken away? Where do i go from here what if any actions can i take? also i'm in missouri.