I am sure your kids would have grown up now. Here are some themes that you may consider for their upcoming birthday. All these themes can be perfect for both indoor as well as outdoor parties and will be equally exciting in both cases. Hope you like them...
Pirate: Can plan for treasure hunt or a swordfight game outdoor in your backyard or any other open space. You can also have a hit and smash piñata game for the boys. Rest of the things such as food, decorations, etc. can be done inside so that you don’t have to bother about weather much.
Soccer/ football: Let the kids play soccer or football game and piñata game outside for some time and do the rest of the partying things inside. You may have the entire party outside in a stadium or may have everything inside by letting them play soccer online.
Camo: This is an evergreen theme for both boys as well as for girls. Let them dress up in their favorite Camo attire and behave like the responsible citizens of the country. This party can be planned equally well in both indoors as well as outdoors.
Here is a link that you may visit for some other useful themes and party ideas.
Hope this helps you.