Why are children not allowed to play a role in adult films, but allowed in R-rated films?

Why children not allowed to play a role in adult films?

Because the law says so.

"In the United States Code of Regulations, under title Title 18, Section 2257, no performers under the age of 18 are allowed to be employed by adult industry production companies."

but allowed in Rated R films?

Because there is no law restricting it. Furthermore there's the practical issue that the rating comes after the film is made, so there'd be no real way to police this.

we all know Rated R films are not suitable for kids

In general, sure, but on a case-by-case bases, that's pure opinion.

The answer to this question really has a lot more to do with how films are made than whether a rating is "PG", "R", etc. Naturally, an "X" rated film does not have kids in it. Most don't even have plots, so how you would integrate children into it is a mystery.

Movies, in general, are filmed in bits and pieces. The focus on a selection of scenes one day and another selection the next day. They do it that way so there is continuity in lighting (for example). If there are a number of scenes outside, they would want to get all of those scenes done on the same day to try to ensure that the daylight was as close to the same quality as possible. There's no way to tell if the sky won't be overcast the next day... make sense?

When it comes to child actors/actresses, the parts the children are in are relatively generic. So, like, the Goosebump movies... the kids know they are not being chased by real zombies (or whatever). And a lot of the scarier stuff is done with CGI and special effects. So, the kids are not afraid or in danger or anything like that. Then the bits and pieces are put together in the 'editing room' and we get to enjoy the finished product.