Ok, I originally thought of this because Path Light was saying he was Chomper's older brother. It just occurred to me that maybe Chomper is NOT an only child.
I mean, what if he DOES have an older brother or sister? I mean, Cera must have an older sibling still around as there's Dana and Dinah. And we never saw Cera's older sibling, yet he/she must exist (or have existed long enough to have the kids).
Also, there's Tricia too (though she's only a half sister). Maybe Chomper got a sibling in between film V and the TV Series.
I mean, can we safely assume that Chomper is still an only child or that he ever was?
As for him not mentioning it, Petrie barely mentions his siblings and the same goes for Ducky. Littlefoot wouldn't have been an only child, had not Egg Stealers or whatnot killed the other eggs and had his mother not died. (Had she still been around, she' and Bron might have had more after film X.)
(And maybe, who knows, maybe Littlefoot WILL get a new sibling. It's possible Bron has found some romantic interest again and that might play into the 14th film.)
I think Ducky's mother may have had more kids later so it's not beyond the realm of possibility that Chomper's parents didn't have more kids after him at some point.
Chomper's species might only have one egg at a time, but that definitely doesn't hold true for all Sharpteeeth, as there were three Fast Biter kids in the egg adventure TV Series episode. (And they all had different eye colors too, while Chomper and his parents share teh same eye color. Odd, huh?)
Anyway, it seems odd that Chomper is the only one of the Gang of Seven that never had or could have had (in the case of the broken eggs in Littlefoot's case) a sibling.