For April or May, should USA be a better choice as the weather should be good. Hawaii & Guam can be an option as well.
Australia and New Zealand are also suitable for 10 year old kids, especially that they can experience the farm stay and natural scenery there. However, the best time is Dec - Feb and not Apri/May.
If your family likes looking at historical things, Silk Road or Egypt can be considered. The historical buildings and monuments are really amazing.
If your family likes wild animals, South Africa is also interesting. You can go to the wild animal zoo, Lost City & visit the beautiful "oint of Hope", Cape Town, etc.