This was caused by toddlers getting the "trait_Umbrella_User" trait. I have a mod that fixes this up now on Tumblr - look for thefreshsims, and click "My Mods" on the top bar.
@Bra1nBlast3dYep that is most likely a bug, I never seen a toddler playing outside in the rain with a Umbrella, I wonder how you can invite a npc toddler over in the first place if they can't speak that great? This is the main reason the Play Date thing is useless to me, is unless you know who the parents are and are friends you can't get other toddlers to join for a play date.
What I did to reproduce was to create a new save with my sim on an empty lot, set the weather to a heavy rain, and then use the summon cheat to bring Evie Delgato (pre-made toddler) to the lot.
