So when it takes me fifteen minutes to prep them for playing outside, I want them to play longer than I spent preparing them for the cold. Most often their gloves fall off or get wet, sending their freezing hands inside within five minutes of stepping out.
A handful of hand warmers seem to hold the kids over for a while longer, and are inexpensive and easy to make when using items on hand.
Heat the warmers in the microwave. I microwave for 15 seconds. Different microwaves have different heating capacities, so I recommend starting with 10 seconds and working your way to a longer time to find what is comfortable for you and your kids.