Kellyn is older so she doesn't usually cry or throw fits. She's actually really easy going and well behaved. I do keep a chart for her that puts a "price" on bad behaviors, so she avoids doing them if she wants more privileges. This works really well for her.
The toddler still does throw temper tantrums, but mostly when she's tired or doesn't feel good. Sometimes we pick her up and try to calm her down, but other times we have to let her be and calm herself down. It just depends why she's crying (if she's hurt or if she's just angry for not getting her way).
With both of my kids I like to stick to rules and make them accountable for their behavior, but I like to be loving and understanding with them, too. It's important an au pair can do the same.