OK, saythatagain - are you SURE it's molluscum? I ask because it's quite unusual for here only to be one spot - it usually spreads. And it's unusual for the spot to get bigger - they usually stay the same size but multiply. And it's VERY unusual for hem to last as long as your dd's have - is it ti,kme to ask to be referred to a dermatologist?
Here are may thoughts on molluscum, for what they are worth.
Conventional doctors say there is nothing you can do - you just have to wait for them to go, but threat them with antibiotic cream if they get infected. They usually last about 18 months, then they go. You will be AMAZED at how often people say "Dd had molluscum for 18 months and the dr said there was nothing I could do, then my homeopath/reflexologist/aura reader recommended a treatment and it was gone in a WEEK!" For this reason, I would regard expensive treatments offered by alternative practicioners with extreme caution.
Also, they tend to look worse shortly before they get better. The looking worse sometimes prompts parentss to try a treatment - and miraculously it works. Once again, after about 18 months!
So ca' canny, as my grandma used to say. But in your case, OP, I woudl definitely want it checked out - it doesn't sound like classic molluscum at all.