I can't seem to get my toddler to blow out cake candles to age her up. I either click the cake playing as her and press 'Candle Help...' and then select the Sim I want to help her, or I use that Sim to 'Help blow out candles'. The adult Sim would pick up the toddler, but immediately put her back down. There isn't any error message like the Sim can't get to the cake or anything, so I'm not sure what to do. I do have CC, but I aged up another toddler quite recently and there weren't any problems. This would be the second toddler I've aged up in my game. I got the Parenthood game pack yesterday, if that helps at all.
It might sound ridiculous, as you've already successfully aged up another toddler, but try removing the CC, it's always the best place to start. Just because it didn't affect one Sim... Least that way, it can be ruled out.
Secondly, have you tried repairing your game (Origin > The Sims 4 > click the cog and select Repair. I'd remove the custom content first, though)?
See if you can now age them up (but don't put the CC back until you've tried ageing them up first).
If that doesn't help, let me know and I'll try to have another think...
If I remove my CC and then add it again, will all of it be back in place (Sims have CC hair & clothes, all of the rooms have lots of CC in them etc.)?