Here's a fun way to keep two age groups busy...
First - the back story. My 3 year old son is superhero obsessed. So much so that my pediatrician jokingly asked me if he owns more than one shirt. The 3 year old was wearing his Superman Underoo shirt, AGAIN. I explained, that no, he has four Superman Underoo shirts along with two batman Underoo shirts and two Spiderman Underoo shirts. Most of them are hand-me-downs from his older brothers, but some are new. Literally, that is all he would wear if I let him. We've come to an agreement that he CAN'T wear a superhero shirt to church, but otherwise he can wear them. He's now fond of wearing the Superman shirt under a button-down shirt so he can be just like Superman OR wear a turtleneck under the Superman Underoo.
My 11 year old daughter loves to draw. One day, I needed both of them busy and SHE came up with this idea. She drew a picture of random Batman images, including Robin, words in bubble letters like Zap and Pow all while the 3 year old told her what he wanted on the picture he wanted to paint. She outlined them in Sharpie marker, then she colored them in with watercolor pastels. She gave the picture to the 3 year old with a brush and water and he had a custom Superhero coloring page.
You could make a coloring book as a gift, if you are looking for a way for the older kids to gift the little ones. You could also use watercolor colored pencils. I've even heard you could make dots using washable markers. What a nice way to get a big kid helping a little one!