Re: Toddler can't blow out candles
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)?
Remove CC.
Repair
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...
SJ