I just want to confirm you are aware of the issues adding any pax at a later date. Most feel you should add them initially, or well before sail date, then remove them if not sailing prior to
your 90 day, PIF date.
Adding a pax later
DCL considers not only cabin capacity, but also the # of kids in several age ranges for the kids club and
nursery. IOW they may not allow you to add your 2 y/o when the number of nursery age kids aboard reaches a certain limit.
Yes, DCL has limits of the # of kids aboard in each age group..

Of course most of us would find this # astronomical, and it may be for the Lab, but not so much for the nursery.
They also have limits to the #'s in each lifeboat station. Both of these can affect the ability to add a pax to a current cabin/ressie.
If there may be a chance for your 2 y/o to sail, add them NOW, IMO. You can always remove them for free prior to PIF date, well .... sort of.
Actually concierge deposits are non refundable so ..... that's one of the twists here.
Will DCL apply the 2 y/o deposit $ to your final payment if they end up not sailing? Do you have plans to sail again on DCL? That deposit could be applied to a future cruise in any cat. Would they accept that possible credit as a deposit for a future cruise booked onboard(OBB)? The deposit should only be around $150. Call DCL or your TA IMO.
Truly trying to help and not complicate things.

I know it would be disappointing to say the least, if your youngest was unable to be added at the last minute.
HTH