入学祝いって英語でなんて言うの？

「入学」＝　admission/entry to a new school/university
「祝いを贈る」＝ give a present/gift to celebrate ～
「孫」＝ grandchild, granddaughter（女の子の場合）, grandson （男の子の場合）

「孫に入学祝いを贈る」＝ give my grandson/granddaughter a present to celebrate his/her admission to his/her new school

「入学祝い」は、congratulatory gift for ～でどうでしょうか？

congratulatory　祝いの
gift　贈答品

congratulatory gift for entering ○○　○○入学祝い
○○・・・ preschool, elementary school, junior high school, hight school, college, university ...etc.

「孫に入学祝いを贈る」
I'll send a congratulatory gift (for entering ○○) to my grandchild.