Good point, plus child actors can be great when the director cares to get the performance needed. Some movies where kids are main protagonists and do awesome jobs are:
Room, IT, Moonlight, The Witch, Wonder, Beasts of No Nation, Oculus, Lion, The Road, Annabelle: Creation, True Grit, Southpaw, The Conjuring, Ransom, Man on Fire, Nice Guys, Beasts of The Southern Wild, Aliens, The Good Son, Stand By Me, A Monster Calls, Donnie Darko, Atonement, Leon the Professional, I Am Sam, The Boy in the Striped Pajamas, The Exorcist, The Emporor's Club, Let the Right One in, 6th Sense.