I have called other people's children beautiful before and they and their parent/s were always very appreciative. I think its ok as long as it is said in the right context.
I called a girl at my kids nursery beautiful because she had the most stunning world book day costume on. My son was saying wow at her costume, and I said to her, "wow, you look beautiful!" And she was so happy and her mum was too and thanked me.
On another occassion, also at nursery, there was a little girl dressed immaculately in the most stunning Eid clothes, and I said to her "you look beautiful in your eid clothes" and she was beaming and thanking me, her mum was very appreciative and said it was nice of me to notice the effort, as well.
Other adults have said my kids are beautiful too, I didnt find it weird at all. However, for instance, if a creepy man randomly approached us in the street to say how beautiful my kids are I think both my kids and myself would be very uncomfortable.
It all depends on who is saying it, in what context, if its someone at least familiar, etc. I just really dont think theres that much of a problem with it in most instances, tbh.