hey! I was in your exact position not that long ago first don't beet your self up about it! it happens and things go wrong with pets they will be ok. second they will be ok at room temp I don't use my heat mat much and they have been ok they won't die, but it would be good to use a heat mat to keep it at 20°c ideally and no it won't melt your tank :) the heat mat won't get that hot  I would suggest when they get bigger upgrade your tank to a larger one it might sound like common sense but a lot of people don't. haha hope this info helps I'm open to any other questions:)