The party thing was planned ahead of time, Sydney specifically wanted her 3-yr-old cousin (SIL's D, they are 6 weeks apart in age and best pals) to come to HER house to play for her birthday. By August (Nathan's bday), it will be up to Dan to host his own parties for the kids with his family, which probably means his mom will host them...
I made the 'executive decision' NOT to make a point with the gift or being overt about our separation during the party. The party is for Sydney. It should be about Sydney. I can distance myself from Dan's life after we have this party....my nieces (3, 13, 16) have no idea we are separated and I don't want to share that with them in the middle of a party.