Also it messes with the other children who see parents arrive and may act out looking for where their parents are!

I too do not allow it. I had one dad who would text me but literally 2min before. I said to him "I don't have my phone on me all day, I am caring for the children. Please give me at least 15min to finish up our schedule and get your dd ready." So he got the hint and would text 20min before :P