i have exactly the same problem with my ex. he thinks he knows it all but stick to your guns. unfortunatly your little girl is still abit young in the eyes of the law to have a say. i would see a solicitor asap if they only have a two bedroom house the boys should stay in the same room when your daughter visits. i would attempt to gain full parental rights perhaps not full custody its a bit different it allows you to have more control of the situation. may i ask what your daughters medical condition is? my daughter has bad exczma and my ex wont apply creams properly. so i took pictures b4 she went for visits and when she returned. she always came back covered. it may seem extreme but if he is not caring for her properly it may be your only option to use this against him in court. if you have always been flexiable and willing to come to an agreement the court will look favourable on you, my ex partner tried to get full custody it was a horriable time and very stressfull but worth it. the judge can tell when you are genuinely concerned and not just beeing vindictive.
ps sorry bout the spelling....
good luck..