It would be nice if some families occasionally brought their toddlers with them, but that introduces the whole constantly-crying-toddler problem that gives everyone bad moodlets. I agree that inviting over the whole household would be a great option to bring back.
There must be a bug in your game because my unplayed toddlers are aging up just fine. It's kind of a pain, but just have one of your parents (or whoever is planning the social event) go around to each house and meet the toddlers. You can check which households have toddlers by going into Manage Households. Once they've been introduced, they can be invited on a play date.
