In the second compartment, another note merely reads, "How youthful are you?"
In the compartment, Zara can see three kinds of diapers. One that looks normal, one that is twice the thickness of the first one and is a light pink with blue runes, and one that is even thicker that screams baby girl with brown runes. There is also a selection of cloths to choose from.

Zara blushes as he beheld the contents of the chest, having not expected this kind of youth. He looked around to make sure nobody was watching and blushed further as he clumsily taped the thickest of the three diapers onto himself, thinking this was some sort of test “prove oneself”.

Zara doesn't notice anything off about the diaper when he puts it on, unaware of the effects. (This diaper doubles the rate Zara makes continence checks, doubles his consumable penalties and auto fails his rolls against consumables or poisons. On the plus side, it gives him advantage on attack rolls against creatures of the dungeon, and immunity to critical hits from them. They still hit, they just don't get extra damage.)
There is still a selection of cloths to chose from if Zara wants some.

Zara grabbed a plain pink onesie and slipped it on, waddling his way to the next room while still blushing harder than he ever had before.

The next room is a storage room, with four bottles to choose from, one large and pink, one that looks normal, one that looks like chocolate and one that looks like banana. There are also cookies and jars of baby food to chose from. For some reason, Zara gets the feeling he should only pick one bottle and either a cookie or jar of baby food.

Zara grabbed the normal bottle of milk and took a cautious sip, looking for a chocolate chip cookie.

(You mean Zara?)
The milk doesn't seem to affect Zara besides the obvious consumable penalty, though it tastes nice. Zara finds a chocolate chip cookie quite easily. (If he eats it, make a constitution save.)
Edit: (Consumable penalty from the milk is -6, including the effect from the sissy diaper.)

Zara licked his lips once he was done with the bottle, starting to munch on the cookie.

(Constitution check. 1+3=4.)

(Welp, nat 1.)
Zara doesn't feel the effects of the cookie at all. (The cookie makes it so every point of damage you take is given to you as a consumable modifier for the next three hours. With a nat one, half of those points are going to be a permanent continence penalty instead.)

(So if I take 4 damage, I get -2 permanently and -2 consumable?)

Zara got up and, feeling refreshed after a nice snack break, headed to the next room.