The warmer the weather the more fun the kids parties tend to be...
Theming the party for Princess, Superhero and Pirates etc, is soooo last decade... why not be a little more Seasonal by having a Water Themed Party filled with little back yard kiddy pools to jump in and splash around. Filling Super soakers and running around. Using up energy on activities that are water based. Such as - See who can fill the water tank the fastest.
You don't have to use a lot of water either - use the water out of your stored rain water tank or even limiting the use of the water around the yard (watering the garden and grass at the same time.)
Utilising Water in the beverages - getting each child to create their own water based Mocktail. Put out a table of Ice Cold Water Jugs, set up Fruit, Cordials or Juices to add plus Ice Cubes in different shapes add a few different pieces of fruit such as Blueberries, halved grapes, quartered strawberries and the kids have their own Non Alcoholic Bar.
Also creating a Mini Olympics Party is fun with all the game set up before the kids arrive you can have someone put up a Mock score board with each of the kids names on with a Cute little trophy for the winner.
These are both Unisex ideas and go down well with the parents for the use of the kids energy making sure they are not coming down from Sugar highs on their way home. You can still have sweets but make them limited filling the kids up on the more savoury items such as home made sausage rolls, mini hot dogs and the like.