Make a picture frame - buy a foam or wooden frame in any craft store, paint (if wood), then attach foam/wood shapes (paint the wood separately first, then glue on). Or stick macaroni shapes on the frame and spray paint the whole thing. Put in a photo of the kids. Or they can each do one.
Or do a little book on "what I love about my mom" - they can write out what they want to say, or they can dictate and someone can type it up, then they can illustrate the pages. You can put it in a store-bought rigid folder or report cover, or make a book cover out of wallpaper scraps, thick cardboard, and colorful duct tape. You can probably find the instructions on line but our schools do this with the kids' stories.
They can also paint flowerpots (plain terra cotta) or you can spray paint them and then the kids can stencil flowers or stick on shapes or use paints for abstract patterns (or do sponge painting). You can add artificial flowers or a green plant in some regular potting soil (if you do the latter, be sure you buy and paint a saucer too. Use a bottle cap to cover the hole to keep the dirt from coming out but allow excess water to drain.)