If you’re looking for an 18″ wheel bike with gears then one option is the Raleigh Pop 18. It’s available in blue and green and is aimed at ages 5 to 7 years old.
It’s worth pointing out that the Pop is about 2 kg heavier than a lot of the other 18″ bikes, so you need to consider whether your child is a strong and confident cyclist able to cope with this additional weight, and the complexity of the gears.
The components on the Pop 18 are mainly steel, so it may rust more easily than some of the more expensive bikes so do make sure you keep it well maintained, especially after riding in the rain.