Personally, I would show compassion.

My DD occasionally wet herself a bit at age 4, having been daytime potty trained from age 2 years and 10 months. I noticed it because she spent all day at a friend's house and wore the same clothes and then wet herself minutes after arriving home with me! I took it as a sign that she needed more of my time! Maybe a protest at being left with my friend for the day (it was my works Christmas do so it was quite a rare occasion!).

Maybe this child just needs soem reasuring that mummy/daddy will be back/is around etc.

I did use a doll as a gift as an initial inducement when potty training but with this type of pruposeful action I would go with trying to get to the root of it and reasurrances.

Lots of praise when the child goes all day with no incidents so they realise that 'good' behaviour gets nice posative responses and time/attention from mum/dad.

How you can convey this in a positive way to relative I'm not sure!

All the best.

One of my dc's used to wee themselves when told off or not allowed to do something. I found it best to clean up without actually speaking about or acknowledging what they had done. It stopped although at worst it would be 3/4 times on a bad day! Child was only about 2 and a bit though.

DD1 did that for a few weeks for me, but still occasionally does it for my mother who takes her a day a week. I completely ignored her, apart from telling her to go and get changed, but did make a fuss of her when she was good.

In hindsight I'm nearly sure it was a response to my Dad being poorly, which I didn't know was affecting her. I think if I could have dealt with her anxiety sooner it might have passed sooner. Now I think she is doing it for my Mum as a protest about me working.

Same old, same old, ignore the bad, praise the good.

The best way to deal with it is no reaction and just clean the child no need for comment or roll of eyes etc (sometimes easier said than done, I know) and lots of praise when they go on the toilet