In Spanish, the most common male gender ending is -o, and the most common female gender ending is -a, so when you want to include both you can use the "@", as in "hola chic@s" (hi girls/boys). I always liked it, but only works for written language.
Either that, or you can just go to the diminutive and say "hola chiquis" (hi little girls/boys) which is not gendered. I use it constantly in spoken language.