Having Party Themes for kids is definitely a popular trend. People have been doing it for decades.
Here are 3 simple things to remember:
1. Get your kids involved so that you chose a theme that they are interested in.
2. Keep things simple: the theme is there to make things easier for you - not harder. Choose 3 to 5 theme appropriate options that fit your budget.
3. When hiring entertainers for your party, remember that licensed character fees can be very expensive so many professional performers have created their own original characters. Ask them what their original characters can do to fit your theme.
Planning a party can be overwhelming for sure, just remember that the point is to celebrate a birthday with family and friends, with good snacks, and some cake. Everything else is just for fun.