Our youngest is a couple of months older than your two and I've only just started to let her play. So, it'll be fine. These playdates are for us AND the children. I'm not too sure when our next one will be but once I know I'll bring you along and introduce you to everyone.
I'll have to see if it helps. You have a number of sleepless nights to look forward to. Unless Paige and Danielle turn out like my Henry did.