Rihanna and A$AP Rocky rented out Sloomoo Institute, an immersive slime experience, in New York City on Wednesday evening, May 13, to celebrate their son turning 4 alongside close family and friends.

The bash featured custom slime-making stations, interactive installations and the venue’s “Sloomoo Falls,” where guests can get drenched in slime, according to a Sloomoo Institute rep. In one especially “Mom-of-the-Year” moment, Rihanna, 38, joined the kids under the slime waterfall as guests cheered her on. (A photo from the evening shared on a fan account shows Rihanna standing next to a child covered in head-to-toe plastic before getting covered in slime.)

Videos from the event shared on the same account show the crowd singing happy birthday to RZA, as well as a glimpse of an elaborate birthday cake complete with the large number “4” and a bucket of slime made of fondant.

An inside source said the party also featured “over-the-top balloon decorations,” per Page Six.

The source added that the family had the “most epic” time at the event.