Ideas
I'm getting this strange thing as well. Using the 64x64 lot in Innisgreen world, Fern & Frond Wading Pond. There haven't been any child/toddler sims on the lot at all. Was a fresh save and no families were ever living on the lot. Have a fairy sim living on the lot running an outdoor flower shop, she only had a fairy home and the Starseed teleporter. The only building I had was a very tiny room to have a few planters for flowers. That room was like a 3x3 I have no idea if its tied to enchanting the flowers or not but I'm finding the little things around the area I'm building her new house in (got married, had one kid and two more on the way), I can't be sure but I've found aprox. 12 of them now through out various points of building and expanding the house. At first I just thought I couldn't place stuff outside because of the uneven terrain on the lot, but when putting up rooms and trying to place objects I got the message about object in the way. The first one I found was when I built the house and moved the garden house and have been finding them at random ever since, so I really have no idea if there are more hidden on the lot or not.
I have tried a completely brand new save with just a random sim i turned into a fairy, bulldozed the lot and flattened it and couldn't find any of the little things. The only mods I use add clothing and MCCC. I don't know if it is tied to it but I had evil gnomes at one point that were messing with the plants and that was some time before I started moving the buildings around to make the house and started finding them. Its annoying but when the message about not being able to place objects come up I just pause what I'm doing and play hunt mushroom by putting up walls and moving them around while zoomed in looking for the highlights of an object that will be sold or moved and if I see one I just put some extra walls in and move them around tell the item pops into the household inventory and then go back to building again.
Its weird and I've not seen them anywhere else but the current lot since its the first one I've been building on so I don't know if they are anywhere else in the worlds. I use all the packs expect most of the kits.