I've made one small change. Otherwise it's fine, in my opinion.
We look forward to welcoming you and your partners and children at our Christmas party, which will be held this year on December 18 from 11.00 to 15.00. We'll be having a picnic in the park across the road.
Please let Lily know by December 9 how many people from your family will be coming; this well help us with the catering arrangements. Thank you.