It could just be toddler diarrhoea Toddler's Diarrhoea, Loose Stools in Children | Patient.co.uk it's quite common and normal - DS2 had it and in hindsight, DD2 had it as well. With DS2 it was quite bad and we had him tested for coeliac disease and a few other things. He did have trigger foods though - no juice or cordial at all and anything with too much fibre in it were no-no's and we had to limit his fruit to one piece a day. He grew out of it at about 4.5yrs.
With ours, the doc said 'they'll grow out of it'. So I took them to a naturopath who ordered a blood test, can't remember what it's called but it checks for intolerance to the main culprits, dairy, nuts, gluten etc. Results showed nuts were the issue in varying degrees, eliminated nuts, took a probiotic, poop firmed up, and reintroduced nuts about a year later. They seem okay now, not that I really looked at their poo once they toilet trained.
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