Another ask to you, professor - what kind of Ghost or part-Ghost Pokemon can you recommend for children?
Well, despite the common misconceptions, most ghost types are perfectly fine pokémon for children, provided the children respect them and care for them properly. I wouldn’t recommend, say, a spiritomb or a mismagius for a young trainer, but plenty of ghost types are as easy to handle as any pokémon.
Many first-time ghost trainers begin with gastlies, since they’re so common and have very few physical care needs; they’re not especially powerful until they evolve, but they’re still a popular choice for a person who’s new to raising ghosts. Similarly, drifloons aren’t known for being powerful, but they’re easy to care for and tend to be fairly docile. They may try to kidnap children, but even very small children are generally heavy enough to not be taken away, so there’s no risk there. Honedges and goletts are also easy to care for, although the former can be dangerous around more curious children-- though it may not be out of malice, a honedge will start draining energy from someone who grabs its hilt. It’s an unconscious defense mechanism.
If you want to mimic the traditional starter system, litwicks, frillish, and phantumps or pumpkaboos can fill the fire, water, and grass categories. The former two are better if the trainer intends to evolve their pokémon at some point, however, as pumpkaboos and phantumps only evolve either when traded or when they become very, very old and powerful. As for a choice between those two, pumpkaboos tend to be better at defense, while phantumps are stronger offensively; it would be up to the trainer to decide which they’d prefer.
Finally, if the child in question doesn’t mind waiting, a pokémon which evolves into a ghost type might be a good fit. Rowlets are popular starters in Alola, of course, but cubones and snorunts can also be good options. Nincada husks can of course become shedinjas, but in most cases a trainer would also have to raise the ninjask as well as the shedinja, so that may not be the best choice for a child.
I would not recommend a mimikyu for a child, despite the species’s tendency towards friendliness, unless the mimikyu in question has been trained to be around children already. Children are notoriously curious, and telling one not to peek under a mimikyu’s costume all but ensures that they will in fact try to peek, and while a honedge whose hilt is grabbed will only drain energy as a defense mechanism, mimikyu will retaliate in anger against attempted peekers.