Power of Attorney would have helped. To put her name on the log book it would need her signature.
The insurance company may just swap one policy for another-but- again you daughters signature is needed unless you can explain the situation to them
If you do get a car registered to your daughter, I take it your son has no insurance or he could have drove it on his own policy. Is this why you are getting the car early, so your son can drive it?
As long as it is taxed and MOTed can you not put the insurance in your son's name as your daughter is away for a year.
It maybe an idea to phone the DVLA and ask about the registratation.
The only other way is to get your daughter to get a lawyer to write up a power of attorney concerning the car