My daughter is nearly 4 and was toilet trained from 2. Has been having relapses every so often. I have had a bit of experience with these things so here is what I recommend for anyone else that is struggling through similar issues
1. It is normal for children to relapse with toileting a few times but this usually resolves by age 4. Most kids only get true sensation of when they need to go to the toilet when they are 3yrs old.
2. Is there a medical reason for them doing this. I would first rule out a urinary tract infection as this can lead to soiling and wetting. And the main one is are they constipated? Most kids who have ongoing issues with soiling are constipated. The lower bowel gets overstretched and the poo leaks out around the mass of poo sitting in the lower bowel. The child is not aware of this happening. this is where you get small amounts of poo often. Check and see how often they actually do a full size poo. Doctor can feel thier tummy to see if this is the issue and use of laxatives from anywhere from 2weeks to several years (depending how long the constipation has been there) will keep on top of this. And sitting at a regular time on toilet each day with knees higher than their hips, and leaning slightly forward.
3. Is there a psychological reason why they are not using the toilet - scared of toilet monsters etc. Is it too cold in there for them?
4. What is you previous responses to your child when they have had "accidents" and could this be improved on with a gentler approach
5. New babies in the house often regress older siblings to "baby like" behaviour
6. Try star charts. Could do one simply for them telling you they have had an "accident", for doing poo on the toilet, or for going a whole day with clean undies
7. Remember toilet training is about teaching them the correct place to go to the toilet. It does not teach sensation to go to the toilet. If your child was previouslt "toilet trained" at a young age, then was this more a programmed routine or did they actually have the full sensations there? Remember these don't kick in till age 3 in most kids. These type of kids may need "re-programming" to recognise the sensation to go toilet rather then go at a certain time of day.
Hope this has helped some of you parents out there or given some "food for thought" at least