If you and your stbx can agree to let the children choose then they can. Legally, there is no age in which a child can choose who they want to live with. If it goes to court, it will be costly and emotional for everyone. If things get ugly between you, they can be caught in the middle, used against each other and may learn to manipulate you both.
The thing to keep in mind is that the children do not understand this. Though they may want to be with one parent they do not want to hurt the other parent. They love both parents and they should never be forced to choose. They may be angry, confused and sad. Pretty much the same that you feel only more basic and without all the knowledge that you have about what emotions are and how to deal with them.
I suggest that you talk to each of them to find out how they feel. Talk to them every day if necessary. Let them know that you are there for them and that you love them no matter what happens. Re-enfoce that none of this is their fault and they can not do anything to “fix” it. I was saddened to hear my oldest stepson comment to his father during an argument that he had done everything to keep his parents together. I’ve also heard them both tell him they hated him. I know that this was not true but it will break any parent’s heart to hear it said. They may not remember a lot of this and what they do remember may not be the way you remember it.
EDIT: I do not mean that I was saddened that he wanted his parents together. I know that all children want their parents together even if they have no memories of them every being together. I was saddened that he thought that something he could do would change the outcome and he blamed himself when his efforts failed. He was 9 years old at the time and the youngest was 6. :EDIT
Let them know that it’s alright to be angry or hurt or sad. It’s alright to want to live with one parent over the other…some children are just closer to one parent. The children need to know that this is ok. They need to know that you want them to be happy.
Read some of the posts on joint custody with equal time. IMHO, the child should not be given the choice because they are not old enough to deal with the emotions. And if there is no abuse, the parents live relatively close to each other, can communicate with each other, there’s no reason they should be without one parent’s influence. But, one parent will inevitably try to get the child/children on their side and will talk badly about the other parent or will try to “buy” their love. This is where letting the child choose can be a bad idea.
EDIT: This is only my opinion and it’s for children under the age of 13-15. After they reach that age, if they can give valid reasoning I think it would be alright to let the choose. :EDIT
I suggest that you contact an attorney for a consultation, get an agreement drawn up and signed as soon as possible. Things can deteriorate faster than you may think during this time and it’s a good idea to have that agreement in place…
Good luck to you!