Quoting LittleBear's mama:" It can be difficult if BD refuses to sign over parental rights, but if he has nothing to do with the ... [snip!] ... and she needs to have good evidence as to why the kids shouldnt go to BD. At least good enough for a judge to look into further"
No... that's now how it is in the US, at all. You're not allowed to sign over parental rights in order to get out of child support. In fact, that's probably why another man (or woman) has to be willing to step up and adopt the child. Typically, it has to be the custodial parent's spouse.
And the will is just a disclosure of that person's wishes upon death. A will drawn up through an attorney is a legal document, but they can be contested in court after the person dies. And that happens allll the time.
OP, definitely talk to an attorney. I know thinking about a will is kinda creepy, but kudos to you for planning ahead. I can totally understand why you would want your kids to stay together, I would want the same thing.