Yes, they can.

If an infant becomes a member, their parent or legal guardian can manage the login ID and password for using the Solaseed Smile Club on behalf of the infant.

*When an infant is traveling on an eligible fare that allows them to occupy a seat, they can earn boarding miles according to the same criteria as adults (12 years of age or older).
If an infant boards without occupying a seat, boarding miles will not be eared.