Please note that some of these questions are specific to Little Windows resin especially ratios/portions. Some resins use different ratios and have different times for the resin to rest before using. These answers in the next two paragraphs are from Little Windows:
Stickiness on hands - use paper towels first, then baby wipes initially, this will get most of it off. Then wash with dishsoap. This is probably easier instruction for them than trying to find a special art cleanser, and will make it seem more friendly. If it gets on any hard surface it can be wiped up with paper towels - stubborn stuff can be addressed with acetone and good ventilation!
The only reasons the resin wouldn't set up after 12 hours (should not be sticky to the touch, but will be a bit soft - will harden completely by the next day), would be that either A) the ratios of resin to hardener were off, or B) the mix was not blended completely. If it's just sticky in spots then the problem is B, if it's sticky all over then it's A. Super important to blend well for at least 2 minutes then let it set in the cup for 5 minutes before pouring, to complete the mixing. Another, well measured and mixed batch can be applied over the first one to seal it, don't apply hardener alone, that will just make a mess of it.
I have also found that the soap that is found in artists' stores is also very good for removing the resin. I have also found that just a bit of baby oil with a wipe gets rid of the tackiness. You can find answers to commonly asked FAQs, videos, and how-tos on their website:
Little Windows link
What I have found important is to make sure that you stick to the 2-1 ratio. Two parts A to one part B. Stir well for 2.5-3 minutes. Steady but not too fast stir so you don't get air bubbles. Stir well and into the bottom so you get it all. Let rest the full 5 minutes. Give it a couple extra minutes to get a bit thicker if you want to brush it on or do a pour for doming or onto a flat surface so it won't go over the edges. If you do that, it is that easy. I was resin challenged until I tried this resin after some show that I was at and decided to try it.