Just got a couple little ones

Question
I got a sweet little girl from a pet store the other day as a birthday gift for my 5 year old.  I also have an 8 year old.  We held her and luved her and she was REALLY sweet, I immediately fell in love.  3 days later she died.  The kids don't know yet, so I went back to the store (trying not to cry) and asked if they had another that looked like her.  They did but this one seems pretty skittish.  I've tried to hold it and pet it and luv it, but it just seems so freaked out.  I put it on my shoulder and it peed and pooped on me instantly.  The other one (and the new baby I got when I went back) seemed so sweet and trusting right off the bat.  Will this one calm down soon and if so, how long?  I did end up buying the babies, the newest being about 4 weeks old.  How should I care for them and make them feel safe?  

Answer
Wait, so you got a second rat and her litter? I'm a little confused.

I'd like to say that, yes, with time she will calm down, but it sounds like it will take some work. Take a look at this for help: http://www.suite101.com/article.cfm/pet_rats/23723