Check for grammar

I see nothing wrong with it.

It would be more common to say "an old kindergarten schoolmate," thus avoiding the possibility that you're still in kindergarten. (check the spelling)

I have found a kindergarten schoolmate on Facebook.

(I prefer "on", and I think "on" is probably the most common choice, but other prepositions, such as "at" or "in", are not impossible.)

Hi,

I think of the term 'schoolmate' as one that is commonly used by English learners, but not so commonly by native speakers.



I'd suggest

I have found someone on Facebook that I was in kindergarten with.



Clive

I suspect "mate" may be more common in BrE. I remember a British friend whom I worked with remarking about another "chap," "Well, he was not a friend, he was a workmate." (Definitely not AmE!)

Of course, "classmate" is common AmE.