I just had this issue myself & I have no clue how to fix it without using SimPE. If you have SimPE, it's an easy fix. BACK UP YOUR HOOD BEFORE DOING THIS! (And I don't promise this won't lead to issues down the road! But doing this fixed the issue instantly for me & I haven't noticed any adverse effects.)
Open the hood containing the boddler (baby toddler) using Neighborhood Browser, then use Sim Browser & find your sim that's stuck, click on them & open their profile, then change their age in the bottom panel from baby to toddler. Once you open your game again everything should be fine. As a precaution, I also cleared my cache (groups.cache & accessory.cache files) & deleted the Neighborhood Manager file before reopening the game (these files can be safely deleted, the game will regenerate them as needed).
If you don't have/don't know how to use SimPE, I don't know of any other fixes, sorry. Resetting the affected sim using moveobjects is said to work, but that did nothing for me. Googling "Sims 2 Boddler Bug" may yield some alternative solutions.
