Continuity mistake: When Alex wakes up after having his nightmare about everyone becoming elderly, his eyes are blue. In the next shot, his eyes are green. (00:01:55)
Continuity mistake: In the shot before Vitaly starts the new show, there are only five dogs when there should be six. (01:04:15)
Answer: They don't make an appearance because they were irrelevant to the story. The decision to go back home would have been made with or without any character introduced in the previous movie, so they were simply not included. Alex's parents would have continued to live in Africa after Alex and the others left.
Casual Person
That makes no sense though because (I know it's a kids movie) but the way they portray it, it makes it seem like Alex doesn't care about leaving his parents and what he just recently realised is his birth home. Like, it feels like there was some necessary dialogue there (probably not for the kids though).