Aqua Orbs come in three types here: enclosed water walking orb, large single orb and the double roller orb, offering some variety to the water wlaking experience.
Safely tethered to land you try to control the ball from the inside as you float across the water, double rollers can accommodate multiple occupants allowing nervous types a sibling or friend. General lower age limit is 5 years, but this is largely judged on height and weight so worth asking if they're close.
It's good physical exercise but also good for sense of balance, logic, basic physics and don't forget it's plenty of fun too. Five to ten minutes is usual but kids really enjoying it have the option of up to 30 minutes.