Because there is a "soon" in the sentence, my preference would be to say "I hope that they won't melt soon." On the other hand, without "soon" (indicating a future event), you can either say "I hope they don't melt" OR "I hope they won't melt." The former sentence would be a comment on the "characteristic" of the item you are discussing (that is, it is a substance that does not melt.) The latter sentence would be a statement that you hope that something that usually does melt will not melt within an unspecified time in the future (perhaps because you have it in a cooler or ice chest, which is not completely effective like a freezer).

Thank you for checking my question Grace!

Maybe the later one is more proper