I somehow doubt his parents are in this one. From waht we've seen, Chomper and Ruby appear to be in the Great Valley and not going on this one.
(Perhaps Chomper is ticked at a certain comment of Bron's. Cough *All Sharpteeth are cowards!* Cough and doesn't wanna help or felt that he wouldn't be welcome (Who knows, perhaps this is when Bron met Chomper only a few times and hence still wasn't that keen on him and Chomper doesn't seem to like being around those who don't like him.. Perhaps Bron actually called him a coward to his face and Chomper is still sore over it. This coulda happened before Brave Longneck Test for all we know.) And since Chomper isn't going, Ruby, who had promised to personally look after him, is unable to go either.
On the flip side, if Chomper DOES somehow end up coming along (perhaps tailing the Gang secretly with Ruby) and his parents DO somehow end up near where Bron is, I can honestly see Chomper fighitng his own parents to help Bron.
Chomper has already tripped his own mother before to help Littlefoot and the Leaf Eaters. Later, he hid them in the fifth movie from his parents and lied to help hide them when they were in that bush.
Also, Chomper coming could be an asset. He could always use his Sharptooth speaking ability to misdirect those pesky Yututyranuses.
Besides, Chomper is a sweet
and wouldn't help his parents against the Gang.
Also, Chomper might not be allowed to go due the mere fact that he is a Sharptooth and some in the Great Valley might question what he'd do unsupervised in the Mysterious Beyond.
Of course, we have three more reasons for Chomper and Ruby NOT to go: Red Claw, Screech, and Thud. As those three are after him and Ruby, bringing more Sharpteeth along to an already dangerous journey is a very bad idea. Plus, though Chomper and Ruby do sometimes go into the Mysterious Beyond, they rarely are seen going out of sight of the Great Valley into it.
Indeed, Ruby may indeed forbid Chomper to go, saying that if she lost him, his parents would be ticked.
On the other hand, perhaps Littlefoot & Co sneak off, without permission, to go get Bron (they've done that sort of thing countless times) and Chomper and Ruby stay behind as they have some reason to, perhaps to keep the grownups from trying to stop them, so they are like distractors to keep the grownups busy.
Or, perhaps they talk to Grandpa and Grandma Longneck and, after discussing why Littlefoot is so desperate to rescue Bron, Chomper FINALLY learns what happened to Littlefoot's mother. Indeed, perhaps at that point, Chomper, feeling guilt for what he is, secretly sneaks off to go help Littlefoot (hence why he wasn't shown with the Gang for a lot of the trailer) and arrives just in time (he'd probably be better at getting up the Fire Mountain than the Gang with that balance of his.) to help Bron.
