My boys are about the same ages as yours. I use a sicker chart and it works very well for my just turned 4 year old and has worked well with him for the past year and 1/2 or so since I started it. I started using it when potty training my oldest. My 2 year old goes on and off being motivated by it, but they have very different personalities. This is how I do it.
They have a variety of small cool stickers and whatever tasks you decide they earn one sticker. My 4 year old earns stickers for helping with chores (folding laundry, putting away silverware, cleaning up toys, etc.) and cooperation tasks. My 2 year old earns them for going potty and cooperation tasks (behaving in the grocery store, bedtime, cleaning up toys).
My 4 year old needs to earn 10 stickers before he gets to a star on his chart or a smiley face. I write the star or smiley on the square where the sticker goes. When he gets to a star he wins something. Things he earns for a star are: going to BK for lunch and play, romp and stomp, pool, baking cookies, finger painting, zoo, any special thing we don't do often works well. My two year old earns smiley faces about every 5 squares of stickers. A smiley face earns him a tatoo, a prize, or a special treat. I have a cupboard I stock up on prizes. My boys love box cars and honestly could never have enough so I load up when I see them at garage sales and put them in my cupboard. When they earn a smiley face they get to pick one car out to keep. My two year old loves this. They both also love the tatoos which you can occassionally find for cheap at the $1 section at Target.
I think this system works well because they have a lot of choices. They choose to cooperate and earn a sticker, they get to pick out the sticker and place it on the chart on their own, when they reach a smiley or star they get to choose the prize(out of preselected choices of course). My boys love it! I plan on doing it for atleast 2 more years before I implement them earning $ for an allowance. I never punish with the chart, meaning I never take away a sticker once it is earned so it is only positive reinforcement. If they don't cooperate they don't earn a sticker simple as that.