*You may be able to build an ongoing lesson about the key Buddhist concept of impermanence and ongoing change using various aspects of growing things. From planting seeds indoors in the winter, (or bulbs outdoors in the fall), to caring for the plants, replanting them outdoors once it's warm, watching them grow, seeing them die, knowing they'll grow back next season (if they are perennial or if you save some seeds); tying in rebirth with the lesson. If outdoor space is not available, some indoor pots of herbs might offer the same concept.
*Even if the monks' alms round is not a part of your tradition, it can be explained as part of the culture and practice at the time of the Buddha and a simple cooking or baking lesson (Bodhi leaf shaped cookies? ;-)) could allow the children to be generous and share with one another with a simplified children's alms round.
*The very famous images of the Buddha's footprints could be made in clay or something similar.
*If you can find some tree branches which have fallen and can be cut to size to look like tree trunks, you may be able to make Bhodi trees with leaves cut from green paper and attached to the "tree trunks" with pipe cleaners.
*You can adapt Valentine's Day to be "Metta Day" with blue hearts symbolizing loving-kindness, peace, and universal compassion and an explanation of what Metta means to a Buddhist and how to do a Metta prayer for loving-kindness.
*You could have the children bring in healthy snacks and do an eating meditation; eating very slowly and mindfully and noting the movements and sensations such as lifting hand, bringing food to mouth, feeling warmth or coldness of food, tasting food, chewing, swallowing, etc. This could be paired with a lesson about the interdependence of all things or cause and effect for plants (animals?) to grow and result in food for us. (If it were me, I'd stick to plants to keep it simple.)
*Venerable Yuttadhammo recently mentioned in a dhamma talk, a simile comparing our moments of sila, samadhi, and panna to beads. As we make more and more of those beads, the string that goes through them and holds them together is mindfulness. I think this could be turned into a lesson for children as they hear a story about each bead representing an act of kind behavior (sila), or a moment of being settled and peaceful (samadhi), or an instance of quietly observing the world around us to understand it better (panna), as they string beads for their own "mindfulness bracelet".
*Does your tradition use Buddhist prayer beads to keep count while chanting? String 27, 54, or 108 beads with a tassel (or larger bead) in the middle to make their own set.
*Paper lanterns are traditional for Buddha's birthday. Here's a couple of samples. http://familycrafts.about.com/od/chinesenewyears/ss/eplantern.htm and https://www.pinterest.com/pin/32299322300026496/
*You could make a paper Buddhist flag and talk about the meaning of the colors. https://en.wikipedia.org/wiki/Buddhist_flag
*Tibetan prayer flags are pretty to make and hang around the room and "are used to promote peace, compassion, strength, and wisdom". https://en.wikipedia.org/wiki/Prayer_flag
*The imagery of our "Three Gems" or "Three Jewels" is beautiful. Maybe coloring cutouts of the Buddha, Dhamma (books maybe?), and Sangha for the kids to color in and paste over brightly colored diamond shaped construction paper?
added The traditional Peanut Butter Pinecones or other simple ideas to feed birds and squirrels in the winter can be tied in with lessons on the first Buddhist perfection of Dāna as well as compassion.
added The Story of the Non Returner who "sold" pots on the side of the road could be brought to life with a project of kids pottery making where the parents leave rice, beans, or lentils (i.e. food that can be donated to a charitable organization) as "payment" for one of the pots. If you think a lesson on non attachment might work here also, tell the children and parents the idea is to "buy" a pot that their own child didn't make. ;-)
For the older children:
*They may enjoy putting together some skits of the Jataka stories (referenced above by Sankha) for the younger ones to watch. (Some might be scary though.)
added *Using colored sand, Tibetan-style sand mandalas can be put together, maybe as a group project. Mandalas always have a deeper meaning. A good article here on one built to incite compassion for all living beings. When completed, mandalas are washed away into the ocean, or poured into a jar to be brought to a source of flowing water, as a reminder of the impermanence of all material things.
Good luck with your projects and in teaching the new generation about Buddhism. That is awesome. :)
Lastly, my crafty friends and relatives highly recommend Pinterest for sparking ideas in virtually any craft category.