Weird rash - appears/fades daily

[RubyrooUK]

My two children (5 and 2) have had a rash for the past three weeks or more. It started with them feeling poorly and they developed a rash - first the two year old and then the five year old.

The rash looks like clusters of little red hives. They appear each day and have gone by the next. They are not bites or spots - no centre and they are not round. They appear mainly on their limbs with particular focus on neck, ankles, arms, legs and wrists.

They don't appear ill but it's not going away. We haven't changed any washing powders etc. The two year old has very sensitive skin but not the five year old.

Pictures below. Ideas? Thank you!

[LumpySpacedPrincess]

Urticaria, dd gets it whenever she is coming down with something.

[RubyrooUK]

Ooh thanks - will look that up, thank you.