How to tell employer they underpaid me? I'm a nanny?

I work as a nanny during the summer watching two kids, four days a week. I get paid $247 every Thursday. This week I only had to work 2 days because they took a short vacation. When I got paid today the father only paid me $98.80. I have no idea how they got this number because the days that I did watch them I still worked the regular hours. Shouldn't I have gotten $123.50? I'm going to write them an email... I just don't know how to address this without sounded rude or accusatory. I don't think they meant to underpay me... they don't seem like the type. I just really hate asking for money- it makes me nervous. Any suggestions or advice would be much appreciated. Thanks!