Sorry to read you are going through this again DB. The point here is simply that arrangements for such a young child are not decided upon by the child. It is our responsibility as parents to make these decisions. If we fail, and rely upon a court to settle the issue, it becomes our responsibility to comply with what the court decides.
I fear you are, through no fault of your own, becoming complicit in Mum's decision to burden the child with this decision. Would it be possible to tell the child it is time to come with you? Asking her what she wants is not appropriate to the situation.
I had an occasion where my child said they did not want to come with me on collection from school. It was shocking and heartbreaking, but a little digging with the headmistress revealed that her hesitance came from Mum's foul play.
I agree with Ash that your ex is pulling strings. I would take this issue head on with Mum. Something along the lines of:
Dear Ex,
Our daughter is not old enough to make independent decisions on what she does and with whom she spends time. As her parents, it is our job to provide instruction. The time she is ordered to spend with me is not subject to her preference and we should make this clear to her. Following our meeting with professionals, our daughter demonstrated that she is very happy with me, if given the correct guidance.
When I next come to collect XXXXX, please tell her: "it is time to go with daddy now." She will greatly appreciate not being burdened with the decision you have explicitly been asking her to make, i.e. "do you want to go with daddy?"
Please let me know if there is anything you think I could do to help things go smoothly when our daughter is next scheduled to be with me. I want to work with you to make things go as smoothly as possible for her.
Yours...
That might not be the right thing to write for your situation, but I think you need to find a way of changing the emphasis here. At present, it seems like you are lacking if handover fails.
I've had times that my little one cried saying "I miss Mummy" when handed over to me. I got her away from Mum and sympathised with her: I know you miss Mummy, you should miss the people you love when they are not there, she misses you too, you'll be with her soon to give her a big hug so she knows how much you love her, its ok this is normal... Giving her permission to find handover difficult worked wonders.
Sorry if I'm rambling, just trying to share some thoughts from times I've been in a similar situation. Hope some are relevant.