Is the foam tee-ball set a good long-term solution for a young child?

No, the foam tee-ball set is a starter, not a long-term solution. It’s fine for introducing a three- or four-year-old to hitting a ball off a tee, but the bat is light and short, and a kid who’s genuinely into baseball will outgrow it within a season — plan to upgrade to a real tee-ball set sooner than you think.