Crickets under 5mm are often referred to as pinheads and are at the most vulnerable stage in their lives. It can be difficult to find a way to hydrate crickets of this size as they can very easily drown. The best way to give them access to water is to give them fresh fruit and vegetables with a high water content such as lettuce or bok choy and replacing it with new pieces every 12 hours. Pinheads have weak jaws so they need soft, moisture rich foods that they can easily eat. If you give them a piece of lettuce or bok choy you will notice that they eat it in a way that resembles caterpillars; leaving small holes all over the outer edges of the leaves and avoiding the tough middle section.
If you decide to give them food such as carrot or potato it is recommended that you grate it as a slice can be too tough for their little jaws to break through to the moisture within. Pinheads should be given a dry food source as well however it should be ground into a fine powder to make it easier to eat. At this size, other insects such as ants can easily become a problem if they get into your cricket container so make sure you keep it clean and in a spot that is hard for other insects to access.