It's illegal in many countries to market unhealthy food to kids, so while they can buy the candy, or ask to be bought it, kids TV channels for example won't have ads for them which limits the negative impact.

You can market toys to kids, but most toys aren't harmful to their health, mental wellbeing, or financial state. Arguably many kids toys promote good health, wellbeing, educational material, etc.

What if it's a game that is good for mental development but has microtransactions, allowing them to unknowingly spend $1000s while also stimulating their brain?

Good mental development includes not being tricked in to spending other people's money, so I think such a game is impossible.

Practically speaking, if 8 year old walked into my store with their parents credit card, I wouldn't allow them to buy much more than a pack of gum.