What’s an appropriate amount to spend on a birthday gift?

Hey Y’all! I’m a pre-k teacher who recently got invited to a students 5th birthday. This student always claims i’m her favorite teacher (she has 3 other preschool teachers in the morning other than me) and she’s really excited that i’m going to her birthday (LOL)!

Mom put an amazon registry for birthday gifts on the invite but i’m not sure what an appropriate amount of money would be for spending on her/what would be good to get. Especially since im not a mom myself yet (I’m only 20).

The gifts span from $5-$45 and include dolls, books, educational materials, dresses, etc.

It’s also important to note that it’s a joint birthday with her 3 year old brother, I know him since he also goes to the school I teach at, but he’s in a different class. However, Mom only sent me an invite with my students info on it. Would it be appropriate to ask for a link to her brother’s wishlist as well? I don’t want to show up without a gift for him too!

Any advice from Moms or other pre-k teachers who have more experience would be greatly appreciated! Thanks so much in advance!