sould kid's play m game's?

According to the ESRB (& the IARC), games that are rated M have content that's appropriate for anybody age 17 or older. ESRB (& fellow rating boards) tend to be on the cautious side & can rate games higher based on one small section of the game. This can cause some inconsistencies where a game series bounces between two ratings, even though the games in the series has similar content.

While stores (usually on their own corporate policies) will card buyers of M-rated games (as their register will bring up a prompt to check) if they potentially look underage (if you look around 25-30 or older, the clerk may use the generic ID to get through the check), it's ultimately up to the parents to make the call on the purchase & playing (by setting the parental controls on the game console or PC / digital platform) these games. Parents have an idea on how mature their children are, regardless of their age, & what content they're comfortable with.

For myself, I tend to set a baseline for my nephews / nieces & friend's children wanting to play some of the more mature games in my library & generally check with their parents (being a responsible adult). GTA V is just one game on my "No Play" list due to it's content. Saints Row series & other games in the GTA series are on the "judgement call" list (usually deferring to parent's wishes). Bully & Retro City Rampage (being Rated T) are on the "permitted" list (generally allow unless parents object) as the content isn't as offensive or objectionable by comparison within the same genre.

Dark Souls series would be on my "judgement call" list, despite being M-rated as it's rating is more violence based & doesn't have the sexual or drug content that the GTA games tend to include. For this series, I take into account the child's gaming history & how well they handle death / failure because of the game's extreme (but fair) difficulty.

My game library runs the gamut of platforms & ratings, where most of the games are in the E - T range... so there's plenty of games for them to enjoy, even when their parents restrict what they can play.

Hope this sheds some light on the subject.

Yes. It is too cruel to not let them play.