Hmm. I suppose you could restring them. The strings on mine look like thick shoelaces, coated tips and all, and the entire piece of string is about 12-14 inches long and between 1/8" and 1/4" thick.

If you hold the castanet in one hand and pull the two string ends tight, then the other "loop" disappears (because it's snug against the outside of the other half of the castanet). Try just loosening it by pulling it out a bit so that there's room to slip it over your thumb, and then the other side should fit over your thumb as well. I'm not quite sure what you mean by a loop inside the castanets; there really shouldn't be one inside, since the string just passes through the top to keep the two sides together.

You can see how the strings should go fairly well in the picture on Carmen de Vicente's home page (the link that someone posted earlier).

Does this make sense?

--Liz