Joined by Moore's daughters Rumer and Scout, the group spent the day at the pink-themed Hollywood Hills home of hosts Balthazar and Rosetta Getty.
"The party was very small and intimate," a source tells PEOPLE. "Ashton and Demi looked happy and totally unaffected by recent relationship rumors. James and Kim were glowing. Despite being about to burst, Kim looked amazing."
Guests of the low-key affair, catered by H.wood and h.wood tea beverages, were served sushi, sliders, beet salad and mixed veggies.
Brook, due this month, tied the knot with Van Der Beek in a private ceremony in Tel Aviv, Israel, in August.