Dennis or Kevin's intentions with the girls. What was he planning on doing anyway?

Presumably all he wanted to do is exactly what we heard he liked having young girls do -- and that is making them dance naked for him.

Although it’s easy to think Dennis had more sinister/rapey intentions, we never saw him take things “too far” with the girls... other than setting them up for The Beast ‘course.... despite having the opportunity to do so. For that reason, odds are his plans were, in fact, only to watch them dance. Was this only because, as Patricia said, he wasn’t allowed to touch them? Maybe.

But in “Glass” [spoilers ahead obviously], Dennis reveals he never approved of harming the girls. He even tried to rightfully resist Patricia’s orders at the end. As such, he may have been a perv of some sort but he clearly had control over himself and so in all probability he wasn’t going to do anything to his captors in “Split” besides making them dance in the nude. And having The Beast feast on them later on.

Based on the hints in the film, I get the impression Dennis is a paedophile/ephebophile and would have wanted to hurt the girls and assault them. Patricia (and possibly Hedwig...) kept him in check, as the three of them wanted to keep the girls in "pure" condition for the Beast - some sort of sacrifice it demanded.