Both the Family Code, Texas case law, and even the US Supreme Court set a high bar to allow 3rd parties to seek custody of children against parents.
One way to lower that bar dramatically is to voluntarily give rights to a 3rd party in an order. In the Interest of S.A.M., P.R.M., and S.A.M. 321 S.W.3d 785, Court of Appeals of Texas, Houston(14th Dist.) shows how. In an agreed order the conservators (who in this case were an uncle and aunt) allowed a friend of the family to have telephone access to the children. The next year the family friend sued the conservators and asked for custody. The trial court initially dismissed the friend’s case on the grounds that she did not have standing. The appellate court disagreed finding that Texas Family Code 156.002(a) allows any “party affected by an order” standing to modify the order. The court defined “a party affected by an order” as anybody “mentioned in the context of conservatorship.”
In light of this decision, it’s best not to include third parties in any fashion in your agreed orders unless you want to see them in future litigation.