They can "stand up" due to like "morning wood" from needing to pee or just randomly (majority of my family members are boys and I have baby brothers) but I wouldn't expect it to happen from sexual arousal because most kids don't have sexual feelings and if they did I'd be worried about their home life.
I think part of what some people find appealing about this manga is that normally Chika would not be attracted to a child and he doesn't want to hurt the kid, so I think people like seeing him trying to protect him for something they both have very little power over. Though...in real life, theres a lot more that he could do. First would have been to talk to the kids parent and explain the situation and have supervised visits to insure safety. As long as this manga doesn't condone making moves on a child, which so far, it doesn't read to me that way (especially with the author making sure to show us him beginning to lose his grip and thus the mom coming in to remind readers that might go with the flow that it is in no way okay). But I'll still have to see what happens...
This page creeped me out: http://www.mangago.zone/read-manga/sayonara_alpha/mi/3e098b64423ab720704fd2bbe8fb5029_chapter-3/24/
Did he refer to a 5 (8?) year old as a 'man?' Wow, I cannot with this story. Cannot wait till he is all grown up.