If parents approve for their child to attend one then there is no issue as far as I'm concerned. It's the parents' call if they want to expose their children to adults concepts. They can make the judgement if they have observed and think their child has the emotional maturity to understand. If it's where a school, library, or whatever institution is bringing in drag queen story hour without parental knowledge or approval then that's a problem and highly inappropriate. It's not for the institution to make those determinations and expose young children to adult concepts.
If I had kids, I personally wouldn't want them to be exposed to adult themes until they were older and could understand these concepts. Let them mature at their own pace vs. forcing things on them when they're not ready. They'll have enough to deal with when they grow up; let kids be kids and enjoy their childhood without the burdens of adulthood.
And no, I wouldn't teach them specifically about drag queens at a young age to preach tolerance. I would have already started teaching them that in general there are many different types of people in the world and to show respect for their fellow man/woman.