I assumed that only citizens get child allowances at all, but you are obviously smarter than everyone else, or at least me, and taught me something new.
I just checked the website- and it says that the savings plan are for any child who is entitled to a child allowance. If you are entitled to that then you get the savings plan too. According to the site, you get child allowance if you are a resident of Israel, or if you live in Israel as a salaried employee with a work visa for at least 6 months. If you move back, child allowance only continues for the first 3 months.
To pick up his money at age 18, he and his parents have to sign a withdrawal form. If it is after 21, only he has to sign. I don’t know if it has to be done in person. But keep in mind, if he only qualifies for a couple of years of payments because he leaves the country, the flight will cost much more than any money earned!