Urchins broadcast spawn once or twice a year synchronously. I'm surprised to hear that the larvae made it through the planktonic stage in your tank (2-5 days). This will not be a constant supply of new animals in your tank like with freshwater livebearers, depending on conditions the adults in your tank may repeat this performance or they may never. I'd say it's pretty cool. You can grow them along for a little while (maybe supplement them so they don't eat all the food in the tank) then sell the ones you don't want to an LFS for credit. Oh and take some pictures and post them hear, I'm sure everyone will want to see them.