Dani, I run into the same problem, so I focus at this time of year on Festivals of Light and use the poem, all the Christmas/Chanukah and non-holiday songs I can find about candles, bonfires, the sun, stars, etc. to illustrate one of the common threads that connects us. You may have to teach solstice celebrations as a history lesson, but don't be surprised if some little one pipes up: "Yeah! We do that too!"