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It sounds like you were playing with the boolprop testing cheats enabled while your Sim was going through age transition. (if the cheats weren't on, you wouldn't have got the error notice). This is likely to have corrupted your game.
Playing with the boolprop testing cheats on for any reason is a bad idea. If you absolutely must use them, turn them off immediately afterwards. Never leave them on for extended periods, and never save with them on.
Turn the cheat off by entering boolprop testingcheatsenabled false in the cheat box.
I don't think you will be able to play with that family again. What has happened is that the game now has a skeleton conflict. It will use the skeleton for toddlers but the animations and behaviors for a baby. The toddler animations which the game tries to run on the baby skeleton will fail, causing the action that was being performed with the toddler to fail also. This makes it impossible for your Sims to pick the toddler up and tend to their needs as you have discovered. If the social worker shows up, she won't be able to take the toddler. However, the save and exit options, which are locked when the social worker arrives, are not re-enabled, because the social worker's routine failed without cleaning up after itself. This renders the lot unplayable, as the game goes into an Infinite loop.
Send the family to the family bin now before the social worker arrives, because you won't be able to after that and will not be able to shut the game down unless you hit ctrl-alt-delete. You will not be able to play with the family again, but at least you won't have to reinstall your game.