Small animal purchases are always risky so do it from a breeder rather than a pet shop. While many petshops do their best to keep them happy, it is expensive and labour intensive to keep small animals in conditions conducive to their wellbeing. Most animals are stressed already and while they may be 'well' in the shop, the extra stress of shifting their home again to your place is enough to lower their resistance to colds and stress related problems.
I am afraid you have no chance of getting the shop to pay your vet bills as it is your decision to take them there and you can not prove that they were already sick when you got them. The skin condition is common with small mammals in petshops and may very well clear up by itself, but it is always better to get it seen to if you are not confident.
If your daughter is already smitten with them is it not worth it to do the best thing for them? Animals need proper care and that sometimes means expensive vet charges. It is a fact of life.