Soft stool in 4 year old (x-post)

I'm wondering if anyone else has this issue with their 4 to 5 YO? DD2 has had soft stool despite having weaned over a year ago (at three). I remember with DD1, her stool firmed up when her nursing went way down around when she was about 3.5 years old and it is normally quite firm. But DD2 has wet, very slightly formed stool even now - so much that there's no way we will let her wipe herself because there is so much wet stool on her bum afterwards.

She's also the one who gets sick more often and for longer than any of us, so I can't help but wonder if that is related. She's been on antibiotics (by necessity, not convenience) several times, although I have tried to counter them with lots of yogurt and probiotics (although more sparingly).

Any suggestions or BTDT - should we wait it out? Do some kids just take awhile to develop firmer stools than others? Or should we be concerned?