I would put these chinese characters on the T-shirt, "什么时候才能像高年级的同学那样有张成熟与长大的脸" or "盼望着假期盼望着明天盼望着长大"

If you want to print it on T-shirt, you may try a picture which has bat（蝙蝠意为遍福）and peach. Those are many styles can be chose as 福寿双全(Having both happiness and longevity),五福择寿(The five names are symbols of good fortune, ample salary, longevity, happy life and great wealth) and etc.

If you want to print it on T-shirt, I suggest this ’慈竹長青’. ‘慈竹’(Neosinocalamus affinis (Rendle) Keng f.) is one kind of bamboo that symbolizes mother’s love. ‘慈竹風和’ is nice for mother’s birthday as well. It’s about the same meaning. And I can give you this ‘果獻蟠桃’ . ‘蟠桃’ is peach. But if I were you, I would choose ’慈竹長青’ . Send my best regards to your mom ‘ 金萱不老’.

You can design a T-shirt with many small word"寿"or you can draw a big character “寿”in the back of the T-shirt

your mum has 100years old？you can make a history of her life ! Congratulation! and happy birthday to your mum ---century woman ---great woman !