The Martian Child is one of those cute, emotional movies with a child in an important role that left an impression with me and is deemed very much worth watching but I'd have trouble explaining why. I'll push myself through this bit of trouble and try. I think I mostly like it because of the cute. I think of it as sort of like Big Daddy but more serious (and without Adam Sandler). Big Daddy isn't a bad movie overall. I've re-watched it so many times before I realised that movies can be a lot better than that, because it's just too cute not to. But it is a typical Adam Sandler movie with the depth and humor of a typical Adam Sandler movie (and with Adam Sandler).
The Martian Child is another foster child/adoption movie about a single, previously childless man. (Why are there so many movies about this, btw?) In comparison with other movies in this category the closeness to reality of its depiction of the circumstances, the process and the foster care system is probably above average. But I don't have any experience to judge that. That's not the focus of the story, anyway.
For me the story is about the child's unique and uncommon personality. I like how it's never really explained what events in the first five years of his life may explain what every grownup in the story sees as peculiar and concerning behaviour and every child in the story sees as weird and sometimes repellent. It may be important for the grownup's goal to enable him to grow into a normal enough person to function in society. (I assume that's their goal and why they are so concerned about his weirdness.) But it's addressed in the movie only as much as necessary.
For other viewers it may be just as much or more about the new father's development and the challange of and challanges that come with fostering a in general and this child in particular. It's about all of that. But ultimately, the reason I decided to watch it again was that it's an emotional story with a cute small child actor acting and speaking all timid while saying things normal people would find surprising, strange responses. So a good story well told is a great bonus, as it were.