Many parents are not comfortable with some of the things their kids do--like 'stimming'*. Learning to join in and NOT make it "a big deal", goes a long way with the child. They feel the respect of their choice.
For example, one of my boys was "stimming" on a piece that spun around. So I helped him spin it. Then I sang a song, "round and round and round it goes until it STOPS!" Then I would stop it. He would look at me and we would start again. When I stopped it, again, I would say (when he looked at me) , "OHHH...you want to see it SPIN!" Because he has SOME language, I would ask him to say "spin". If he didn't, but was still looking at me and gesturing for me to spin, I would spin it. (Different from ABA--they would not spin it until he attempted the verbalization). I would spin it fast, saying "It is going FAST!" and the slow it down, "It is going slow". Eventually I would ask, "Do you want it to spin FAST or SLOW?"--again trying for language, but not demanding it. (With older kids who are non-verbal, I will sometimes use my hands for them to 'choose"--i.e. put out right hand and say "FAST" and the left hand and say "SLOW". Whichever they choose, I honor the choice.) Again, by joining in on a stim, it will eventually fade, NOT get worse (which is what parents fear). By forbidding the stim, anxiety is fostered and the stim becomes more important to the child and will last longer. Stimming is a way for the child to "veg out" and many parents are too afraid that if they allow it, they will 'veg out' forever. Not true. I can always tell when a certain behavior is not permitted by parents because the child will be so surprised and relieved when I join in. Almost always a guarantee for some level of eye contact. It is usually a sideways glances like, "REALLY? You're letting me do this and YOU'RE doing it TOO??" They are always "ready" for me to take it away--like I'm pulling a cruel joke on them. Once they realize I am there for THEM, it is the first step to building a solid relationship. After that, it is easy. I do not do a lot of correcting or anything else that "puts upon" the child until I feel I do have that solid relationship. Once that relationship is built, I can push for language, socialization skills, and behavior changes (although many of these come naturally, once the relationship is built and the parents are "on board".)
So, join in on that stim that you hate so much. Change it up a little. Show your child what ELSE can be done with that same thing. The trust you will build is worth it.
*Stimming: using an object to retreat from "our world"--usually involves repetitive behavior. Some examples: spinning something (or anything) over and over, just going through books without even looking at the pictures, repeating the same words over and over, playing a part of a video over and over, lining things up, over and over again. The commonality? "over and over again".