How do I stop mould in shed

What is it made of? Are you in a humid climate? Is the finish painted or stained? Normally I would say to wash with vinegar.

kilz

I would definitely start by getting rid of the mold by using:

Bleach containers often have the needed ratio if not a quick Google search...

OR ODOBAN, is fantastic!! A $10 gal will last a year I bet. It kills, disinfects, cleans, and degreases everything and anything, viruses, bacterias, fungi, and more!!

OR... with KILZ, it gets rid of mold by sealing it.

Then, I would move it, if possible. Where it is located could be the major issue, unless it came with some spores. Try an area that gets more early and long sun, perhaps. If you cannot move it, flip it when not in use, if it has an open bottom. Lastly, have the girls go in with rags, or cut a towel between them, and let them dry it each day/use. You want to keep the moisture down and the girls can learn a powerful lesson/responsibility about mold and keeping their own house tidy.

JUST DONT LET THEM PARTAKE UNTIL THE MOLD IS GONE!!

If it possible to install a small fan to keep the air circulating this should help to lessen the problem.

Follow the answers above to kill the mold. Then paint over with kilz. Go over the entire outside and seal any openings with a silicone sealer. Check the roof for leaks. Raise it off the ground with some type of support. And has someone suggested, you might need to move it to an area that gets more sunlight.

Thanks to everyone for their suggestions I will give them a go