What can I use to safely seal the painted design on this cake plate?

Hi Patricia, it is my understanding that there really isn't any sealer that is food safe. Most ceramics are fired at high temps to bake in the patterns, but simply heating a sealer or paint will not make it non-toxic. I have seen painted pieces that are food safe, but the painting is done on the reverse side of the food surface, so the food never comes in contact with them. I have also seen people put the food on a plate and then put the plate on the decorative serving piece...again, so the food doesn't come in contact with the paint. Sorry I'm not more helpful. Perhaps someone else will have a suggestion though.

Check this product out:

https://www.amazon.com/Clear-Food-Grade-Silicone-Sealant/dp/B0063U2RWU

Practically all sealers are not food safe to be in contact with food. If the cake plate is glass or porcelain you can bake the paint on. Place the plate in a cold oven on a cookie sheet. Turn the oven on to 300 degrees and let it bake for thirty minutes. Turn the oven off and let it cool down completely. Do not remove the plate. Once cooled the paint is bonded to the plate.

Mod Podge might make a food safe sealer. Some epoxy resin is also food safe.

Hi Patricia, it looks like you're gorgeous plate has a lip around the edge. You could pour resin inside. This video will show you how - https://www.youtube.com/watch?v=zso4HLxYqjU