parental rights

What is the name of your state? Texas

My husband and I have two children ages 10 and 8 living with us for the last two months. The parents who is my husbands cousin and his wife wrote and signed a piece of paper saying they give all rights of the children to us. The parents now have been sentenced to 30 years for him and 50 years for her in prision. We now want to know what we need to do to have the agreement legalized or even to actually adopt the kids. Can the parents rights be taken away so that we may do this? Do we have to contact an attorney or may we achive this on our own? We want to make everything legal because we do not want anyone else to try to take the kids from us later if the parents are to decide to change there mind.
Thanks
Kim

The parents who is my husbands cousin and his wife wrote and signed a piece of paper saying they give all rights of the children to us.

What was signed? Was it a Termination of Parental Rights? How did you go about taking care of the children - through the government (i.e. Dept. of Children and Families) or was this a private agreement?

The parents typed up a piece of paper that says they give my husband and I guardianship of there children. They then signed it and had a notary sign it as well. When we received the children they were living with there grandparents but because of some health issues and other reason the Department of Protective Services were about to remove the children from that home and place them in foster care. We spoke with the case worker in regards to the children and she said that the letter that the parents signed was all we needed to get the kids in our home. Since the children are now in our home the case has been dropped and there is no longer any government agencies involved. At the time that we took the children the parents had not been sentenced so we were pushing for permanent placement in our home but now that we know the parents will be in prision even with parole at the very least until the children are adults themselves we would like to do all we can to keep them in our home and give that a sense of belonging. I hope this answers more questions.
Thank You
Kim