The gate agent or FA will make it work, perhaps at risk of slight inconvenience to others:
Your wife and 2.5 year old will be seated together. If AA can't do it at T-48 it will be done on-board. Nobody should be tool enough to separate a toddler and mother.
I don't know that every E90 has 3 air masks per side in every row. (Delta & UA CR2s do not.) You and your lap infant will be moved into seating that does have 3 masks.