Child Abuse: Proving Mother Unfit

My question involves child abuse or neglect in the State of Tennessee: I'm 27 with 3 children of my own, 2 girls (6 & 7) and 1 boy (2 months). I also have 2 soon to be step children, 1 boy (11) and girl (7). I'm highly concerned for their well-being and not sure how to handle the situation. They come to our house every weekend and are with us any time that they are out for school breaks. I started asking them questions not too long ago because their hygiene wasn't that great and when they told me that the only time they take a shower/bath was when they were with us, I informed their dad. He doesn't know what to do because he fears that if we try to get full custody it could back fire some how and we will lose our visits with them. Two weeks ago my step son had came over with a busted lip and a hurt wrist. After an hour of talking with him he finally broke down in tears wanting to tell me a secret. I agreed to keep it between us but he then told me that his mom had hit him because he was being bad. Luckily our security cameras caught him saying that and his dad heard it so it didn't look like I betrayed his trust. The kids have also told me that they do not have food at their mom's house and they will ask if they can take some home so that they will have something to eat, which I allow them to do. Keep in mind that the mother does get help from the state, she receives food stamps and a welfare check. How am I supposed to handle matters like this? I have never been in a situation like it and could use a bit of advice please

Child Abuse: Proving Mother Unfit