mould on walls

Ours is a ten year old house and lately we have had some mould appearing around the windows but more worrying is in the back bedroom on the outside wall large spots of mould have started to appear all over the top 2 feet across the whole wall. its quite faint and you need to look closely to see it but is this normal?
The gutter is not leaking, there are no pipes in the wall.

Our two little boys sleep there so a bit worried about their health.
Any advice much appreciated on what may cause this and how do I get rid of this.

We had similar issue but think it may be previous owners not opening the windows.... house now approx 17yrs...we got a mould spray in woodies 7-8yrs and it seemed to kill but now see small bits coming back so not sure if windows were the issue as we open windows daily (so will be interested in this post to see if anyone has any permanent solution to it)

3 sources of damp:
1. Rising damp, not a problem upstairs
2. Penetrating damp, i.e. gutters leaking. Are you 100% sure they are not leaking/
3. Consensation. Its probably condensation. More ventilation, stop drying the clothes on the rads. leave the extractor fan on etc

I would go for item 3 above, you will notice the problem getting worse as the children get older not their fault ( this time ) humans exhale moisture as we breathe, the bigger the person = more moisture per hour. Check the ventilation in the room, if a wall vent make sure it's not blocked, if trickle vents in the windows make sure they are open. If there is no visible ventilation it would be a good idea to look into installing a vent, the type of mould you describe is really warm moist air meeting the coldest point in the room, the black marks are usually airborne dust standard in every home.