Hope ~

I have one minor suggested edit:

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I’m planning D4’s party and as you know it’s very important to her that we celebrate together. ,so for her sake I still think that’s best. Please let me know if you would like to join us. I’m planning and taking care of XYZ, please let me know if you have ideas or anything you’d like to contribute or incorporate.

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Standard co-parenting advice is to keep it business-like (read: detachment) and always focus on the needs of the children first.

Your response is business like, and it focuses on D4's needs (wanting a family party). Perfect. You can always change how you do these family things in the future.