There is yet another altercation with the main characters and the neighbor and her dog at about 30mins in. At 36:40 M3gan gets the idea to hurt the dog. at 36:50 it starts, M3gan calls the dog using its owners voice to the hole in the fence. She drops something. The dog comes to investigate and gets grabbed. The movie cuts then to the morning. At 1:01:20 we are told he is under the ground somewhere. We never see it.
A child is not physically abandoned, but part of the plot revolves around Cady being emotionally abandoned by her guardian (aunt) after her parents die. Rather than raise Cady, she gives her a prototype doll to take care of all of her needs. It is even brought up by another character that she is not being a parent to the child, because she's given him a surrogate to raise her.
About 25-30? minutes into the movie, there is a scene in which M3GAN is shown watching Cady play outside with a bow & arrow. A rather large butterfly (or moth) lands on the outside of a glass surface where M3GAN is standing, and the audience “sees” the butterfly/moth through her eyes.
Also, around the 42 minute mark, Cady tells M3GAN a memory about her Mom & the memory mentions a cockroach.
There is a scene with implications. An older boy with violence and other behavioral issues takes M3gan to a secluded spot in the woods, where he throws her on the ground and begins to undress her (Her show is removed.), before M3gan attacks him. There is an implication that if she had not attacked him, he may have tried to assault her. She is not a child or even human but has the appearance of a pretty pre-teen girl.
She is not "beaten up", but a boy in her school forcibly presses the spiked casing of a chestnut into the bare palm of Cady, making her scream in pain. He only stops, when they are interrupted.