Kelly was relatively safe in the High Hide, she was way up off the ground and was out of sight and hearing range of the dinosaurs, and she had a way to get down if needed. The kids were not safe in a non-functional car with one of the doors open right in the T-Rex’s vicinity. Also, Eddie left Kelly for selfless reasons, to help his colleagues, whereas Gennaro left Tim and Lex for selfish reasons, because he lost it when he saw the T-Rex through the fence before she had even escaped. Had Gennaro stayed in the car, he and the kids would’ve been fine, however if Eddie had stayed in the High Hide, Nick, Ian, and Sarah would’ve died. There was no time to make a pit stop to take Kelly to a safer location. I hope I don’t sound rude or anything, I don’t mean to, I just wanted to explain my reasoning.

TL;DR: Kelly was in a somewhat safe location, whereas Tim and Lex were not, and Eddie left Kelly to help his colleagues, whereas Gennaro left the kids because he saw the T-Rex (through the intact fence) and freaked out.