Amber is just a conventional petite/loli heroine. She's very obviously of the same teenager/young adult ballpark that all main characters and bachelor(ette)s are in the series.
She's clearly written differently from the actual literal child characters the likes of Monica, Candy and Suzu.

Amber is okay for to have more intense feelings for, but I think you might be a bad person if you look at Monica, Candy or Suzu and feel something else other than wanting to ruffle their heads and maybe giving them a lollipop before sending them off their way.