Need help from someone creative?!

I know some people claim these parties are selfish/waste but for my family this is what we've decided to do. All of my family lives out of state and will be coming in for a family reunion in March, a month after our 18 week u/s. and we thought it would be a great way to share with both sides while there here instead of phoning my sister/mother with what the gender is. Anyway, for our party we are keeping it low-key and economical by doing a sundae bar around 7:30-8. Of course we would be hosting and provide the stuff. We plan on having blue and pink candies, sprinkles, cherries, sour worms, nuts, etc. The only thing we are stuck on is how to reveal the sex keeping with the theme. My mom suggested colored whip cream but we want to keep it a secret not just pull out a blue or pink whip cream. Any creative ideas sticking with the sundae bar theme?