nomorechoc - I take DS1 and DS2 in the WC cubicle in the male toilets. They want to use the urinals as they want to feel grown up but because they are not really tall enough for adult urinals it is safer to use the WC. Occassionally public loos have urinals that are lower down for children but rarely so.
I have never found holdig them up actually works very well. They just cannot go for some reason. They are often not clean either. I would be extremely relucant to let them go in on their own - except in a restaurant which I know well.
If you are a woman I would take them in with you until they are age 8. Otherwise, I would go in the disabled loo with them. As long as there is not a disabled person waiting - it does no harm and boys are usually fairly quick.
Poos out in public loos are no trouble but again I never let them go on their own.