I think you're being too uptight (and I mean that in a nice way!)
You child's school is now (along with home) pretty much the most important place in your young ones life right now.
Relationships that may last for life are made in the playground, life lessons learnt and (hopefully) a desire to discover, grow and expand their horizons is being discovered as we speak (well maybe not it's half term!).
A really important part of that is social discovery, meeting other people in a "safe" environment and realising that the big wide world is much more than home, the park and grandparents.
I LOVE playdates, much like dinner parties you tend to reciprocate and soon you won't have to pick your son/daughter up some days as they'll happily trot off to another house. That's free time or quality time to spend with your other offspring.
As for safety it'll be fine. If they can raise and child and get them into the same class as your little one then they're probably OK.
There are horror stories but they tend to be about stuff like catching nits and cultural differences ("Why don't we say Grace?" "Why don't we watch TV in the living room with our dinner on trays"?) than safety.
So don't worry and watch your little one grow, I envy you, it's ace to see them to explore the big wide world!!!
ps hope none of this comes across as having a go at you, it's not, I love your posts on here!!!