Chicks with runny bum

Don't apologise for the picture.  Your chicks are lovely and it illustrates very clearly what you are trying to describe. 
I am not an expert and I didn't see this in my two chicks but I am wondering if this can be the end product of digesting the yolk sack.  Chicks can go on digesting their yolk sack for 2-3 days after birth.  I'm sure someone with more experience will be along to advise you.  In the meantime good luck and keep us posted 

Thanks both, it does tend to get rubbed off with their scratting around (the pic was a freshly squeezed one!!) and i don't really want to handle them unless I really have to as it really upsets them all

I did wonder myself whether it might be to do with the yolk as it looks a bit yolky ish but just worried it may be something more sinister   I will keep an eye on it today and if it is the yolk presumably it should clear up pretty quickly.  Thanks again.

Seem to be back to normal today....panick over