Ok, you’ve established that the kid has pinworms (enterobius vermicularis). Mebendazole is an easy way to treat it. You only have to give one dose, then repeat in 2 weeks. What could be simpler?
The tablet is chewable. For some kids, that’s fine, just hand it to them. For other kids, that’s not going to work. For them, you have lots of options. Mebendazole works fine if you crush it up and mix it with food. One of my favorite tricks is the candy bar approach. You cut off the end of a soft-centered candy bar. (Three Musketeers® works great.) Stuff the pill in the end of the bar and give it to the child. Let them see you’ve done this, they’ll figure it out fast enough when they take a bite. But you can then dangle the remaining portion of the candy bar right in front of them. The classic bargain—you eat that and I’ll give you this. As you can imagine, this works fairly well. Just make sure you put that remaining candy bar directly in front of them so they can see it and smell it. Leaving it on the counter takes away some of that incentive factor.