Kardashian was spotted leaving Sky Zone Van Nuys, an LA-based trampoline park, Wednesday evening with future stepson, King

By Melody Chiu and Karen Mizoguchi
Updated April 29, 2016 01:20 AM
Prince Williams/WireImage; Source: Blac Chyna Instagram

The fatherly role is a responsibility that Rob Kardashian is eager to take on.

The reality star was spotted leaving Sky Zone Van Nuys, an Los Angeles-based trampoline park, Wednesday evening with fiancée Blac Chyna‘s son, King Cairo.

The pair arrived around 5:30pm and were seen leaving at 8pm. Joining them was a mystery woman, believed to be the 3-year-old’s nanny. An onlooker tells PEOPLE that they were having a blast together, with Rob holding King’s hands while he jumped on the trampolines.

The eyewitness described Kardashian, 29, as being a “doting future stepfather.” King Cairo is the son of rapper Tyga, who has been dating Kylie Jenner – Rob’s half-sister – after ending his engagement to Blac Chyna.

After news of Kardashian and Blac Chyna’s engagement went public, Tyga took to Twitter to wish his ex-fiancée well. “It makes me happy to see the mother of my son happy. My only concern in this situation is my son. I want him in happy environments,” he tweeted, adding, “He’s innocent in this. & I want him to feel as much love as possible.”

Kardashian has been spending lots of time with King Cairo since dating his mother in January. Earlier this month, the engaged couple surprised the child with a trip to a local fire station, and in late March, they also celebrated Easter together.

Kourtney, Kim and Khloé Kardashian‘s brother even celebrated his March 17 birthday hitting up Legoland California with his soon-to-be wife and stepson.