To be fair, Esther is the one carrying all the medical risks and could have potentially died as a result of having the baby. Although I hate the surrogacy storylines because the surrogate always keeps the child, posters complain about characters immediately accepting their biological parents and ignoring the people that have raised them. Esther has 'raised' the baby so clearly has some rights. I don't see why they can't work out an arrangement where the baby lives with Grace and Trevor and Esther gets to see it; or even a three-way parenting.
Even though the baby is biologically Grace and Trevor, what about when the kid asks about her being pregnant or her giving birth and they tell her there was a surrogate? Whichever way it's presented, there are essentially three parents.