Ex trying to change current child arrangements.

I recently received a letter from my ex’s solicitor stating that the current 50/50 isn’t working. Reasons being my eldest 7 year old show distress at handovers and has said she wants more time with mummy in school week. Current schedule is wed evening I pick up and Sunday 6pm return. Next week is wed evening I pick up and return Sat at 10am. She requested one Friday a month she picks up from school and youngest from nursery and I agreed to this. She is now suggesting that I have every other weekend. She plans to implement this in January after the Christmas break. I note the timing is advantageous to her as solicitors and courts will be closed over Christmas and new year period. My eldest also show upset when I drop her back on my days. She is adjusting. This has been since June. I entered into a period of no contact to protect myself and avoid conflict. All communication is via my mother and so are drop offs. She has lied and said I have blocked all communication which is untrue. I only blocked WhatsApp and social media. All lines of communication have remained open. It’s work well going through my mother and avoided unnecessary arguments. The child’s mother often cries at hand overs making the situation worse. I’m assuming no mediation will have taken place before she implements this. Is now saying the agreement was imposed upon her but I have a written document from her laying out the 50/50. I’m at a loss of what to do. I can get a solicitor but it’s leaving me literally weeks before she will implement it. Can I stop her?