The reality star and her 2 children with Tristan -- daughter True and son Tatum -- popped up at the Rocket Mortgage FieldHouse Monday ... where the Cleveland Cavaliers are set to face against the Boston Celtics for Game 4 of the Eastern Conference semifinals.

Sources familiar with the situation tell TMZ ... this game is extra special as it's the first time True and Tatum are seeing Papa Bear play professionally. We're told True has been asking to go to a game ... and so Khloe wanted to make this a special outing for her.

While Tatum may be a bit young for the sporting event, sources say Khloe thinks it will be fun to see what he thinks of his dad playing.

Ahead of the big game, Khloe was spotted wearing an all-leather look as she held her son Tatum's hand. Tatum, who is only 21 months old, seemed to be walking pretty confidently as the family made their way into the arena. True chose to wear a bright pink t-shirt and sparkly boots -- forgoing the Cavs' signature colors wine, gold and black.

Khloe also played chaperone to her nephew, Saint West, who joined her kiddos for the big night out in Cleveland. Kim Kardashian's oldest son also chose to support Tristan by repping his jersey at the game.

Despite Khloe and Tristan's rocky relationship in the past, the twosome have been really great at co-parenting. We're told Khloe always wants to make sure she helps in ensuring the kids spend as much time with their father as they can.