I put a similar post a few months back, but ds was only doing it at nursery (he had moved into the pre school area and I did wonder if htis was him "attention seeking?)- following 5 months of potty training (which came really easy to him).
Like vivs we were getting a bit annoyed with him which in turn made him really upset, so we gently tried to remind him to go to the loo and also avoided the potty as at nursery in the preschool room they only have toilets. It took about 4 weeks with him but success has been achieved again.
Have either children had any changes to their normal routine?????
Persevere and it will come back