While subjective, most of these
should be accessible to younger and less-experienced gamers, though some normal ones would be suitable too. There exists a system of tags that can better help you filter results down to the right stuff. "Traditional" is a good tag if you want more like Super Mario World. There are a ton of nice short easy hacks, like Oogtar's Quest, super Mario's Island-Hopping Adventure, and Sunny Side World (this one is in Japanese but you do not need to be able to read it to enjoy it.)
Hacks I can recommend individually that are lengthy and should still be kid-friendly, and might be listed under normal, are:
"Return to Dinosaur Land" by RAGB
Any of the "Super Mario World: Master Quest" games by Alex No
Gamma V's hacks also from what I hear are very balanced, but I haven't had the fortune yet of trying most of her hacks.
I think with the exception of the exclusively one-player hacks, the other hacks here are going to be two-player alternating turns. The concept of simultaneous co-op has been toyed with in the past but I believe it wasn't ever fully completed. Some SNES games (not hacks) you would have a blast with that are kid-friendly are "Joe & Mac 2: Lost in the Tropics," any of the 3 Donkey Kong Country games, and "Kirby Super Star" if simultaneous co-op is a must.
What platform do you intend to use? Ideally, a SNES or super "Famiclone" and cartridge to dump the hack to would give you that comfy couch experience but it obviously comes at cost. Some emulators support netplay if you intend to play on separate computers or remotely. The easiest way to play together if you are both in the same house, on one machine, would be to select a 2 player file and take turns sitting at the computer. You could also use a single player, but be sharing lives and powers.
Snes9x with the option to use unlimited echo buffer size would enable old hacks to work if you run into that problem. ZSNES also has netplay but will not properly support anything with SA-1. Higan/bsnes may need to be run in compatibility mode for older hacks and is a more CPU-intensive emulator as it uses as many of your free cycles to accurize emulation as possible.
Just look above you...
If it's something that can be stopped, then just try to stop it!