I thought @kirby007 was doing some charity stream 

but I 2nd the Epic Yarn

It has a 'mellow mode' where Yoshi has wings that allow him to fly by keeping the A button held down, making most platforming sections overly easy.  But you can still die since Yoshi can't gain elevation while flying and if you aren't high enough to reach any platforms there's nothing to do but let go and fall into the bottomless pit and start over from the last checkpoint.  You also still take damage from enemies.

There are Nintendo games which have a feature called 'super guide' where if you die 5 or 8 times on a level the game offers to have the level play itself, allowing you to move forward in the game, and this can be done on every level. Of course, you don't actually play the game when doing this but just watch the game.

https://www.mariowiki.com/Super_Guide

Yeah, Epic Yarn is the most easy and friendliest game in the series by far.

The Forgotten Land kinda gets legit challenging after the first 3 worlds I'd say. Don't know if your child will be able to solve most of the side content challenges too because those are time based.

Star Allies might actually be the best point of entry for newcomers I'd say.

It really depends if they're going to play alone, or with you.

I think people are confusing can't die with easy for kids to play. Four is very young and even the easier Kirby games have parts where it's hard to advance without doing some pretty specific stuff. Epic Yarn is particularly hard in this regard since Kirby can't just fly everywhere. I actually think it's one of the tougher Kirby games for a very young child to get through. Yoshi's Wooly World is actually better in that regard, since you can play as a flying Yoshi, so you don't really have to worry about platforming.

I think the way they'll enjoy it most is if someone else is controlling Kirby, and they play as a helper or side character. You'll always be able to make sure they're progressing, and they'll still be able to contribute and have fun. Kirby superstar is great in this regard, since it's a lot of times more fun to play as the helper than Kirby. Return to Dream Land works as well, as does Star Allies. Epic Yarn in 2 player mode is a great choice because most of the game IS very simple, and they can warp to you (no matter which character they're using) if they can't make a particular jump. Forgotten Land works too.

If they're playing alone, the original Kirby is the best as long as they're not turned off by the visuals. There's no real puzzles or anything, and you could plow ahead. Although there are a couple of bosses that will be tough. Adventure is also pretty simple, and for the most part superstar, although the last part of the game has some puzzles.

But main point is that if you're going to play with them, pretty much any of the mainline console games (Dreamland 3, Superstar, Return to Dream Land, Star Allies, Epic Yarn, Forgotten Land) will work well. If they're playing alone, basically the further you go back the better.

get the n64 version on the eshop

Yeah, you almost literally can't die in it. It has a pretty cutesy appeal as well so young kids will probably love - and I say that as an adult who also found it charming.

I don't know about a toddler playing it, but the very first game almost plays itself. I still enjoy playing it, but man... it was really easy.

Thank you. This is very helpful. 

Epic Yarn (Wii) or Extra Epic Yarn (3DS)

There's literally no way you can get a game over in the game and its easy in design.