Some Maltese have separation anxiety, which is definitely something to think about for a dog for a kid, as that kid will be going to school, which means they will be apart from the dog for long periods of the day. The dog may not react well to this.
Pugs where originally breed for royalty in China, to then be found with royalty in Europe. They were meant only as companion dogs with no other purpose and as such are quit chill. They tend to sleep a lot and are very attached to their owners. While needy at times they are definitely a good breed for those that want a relaxed dog that will spend all of its time with the owner.
This also means they are good with children as they tend to be a relaxed dog as well as a bit meaty, in that a small child can not always easily pick it up, which could potentially hurt a smaller more frail dog.
Some poodle breeds are not bread with temperament in mind. This means without research there is a chance of getting one that may be snippy or bite. This of course would be a bad thing when considering a dog for a small child and is something to consider when making the choice of best dog for a child.
Chihuahuas are often small sized and have skinny legs, this makes them easy to pick up, but if a small child does not know yet how to handle a dog, could lead to potential harm to the dog.