I'm not going to high-five a kid at my work
Why wouldn't you high-five a kid at your work?
It's a kid and I don't want anyone to get the wrong idea.
Setting personal boundaries in your place of work is important and you should not do anything that makes you feel off or uncomfortable. A high-five with a child is probably an okay thing to do in most situations, but if you feel it's not then go with your gut and refrain.
But what if he keeps trying to high-five me?
If he keeps trying then you should let him know that you aren't comfortable with high-fiving people at work. There's no harm in letting people know that you don't like to engage in certain things in the workplace.