We like to include the kids, so they painted our Lenten box. Then we wrote a Bible verse on it (John 3:16) and put 40 little boxes on it so that we can check off each day as we add the food.
I think this is a nice way to mark Lent with little ones. Giving up something for Lent is not developmentally appropriate for small children, but this is a simple, tangible way to mark the 40 days, and focus on the giving part of the season. The message I stress with the kids is that we give to others as a reflection of the love that Jesus gave to us.
I also found a neat printable Lent calendar here that we have used. It is another way for kids to visualize the 40 day season of Lent. Enjoy!