Probiotic supplementation in childhood serves as an additional source of bacterial colonisers and represents an opportunity to beneficially manipulate the intestinal microbiome. Differences in the ability of probiotic strains to colonise the gut may be related to the variously diversified gut microbiome. We report the results of the association between composition of the gut microbiome and the colonisation capacity of the probiotic strain Escherichia coli A0 34/86 (CNB – Colinfant New Born supplement) in the cases of three healthy children in different development stages (infant, toddler, and pre-school), as a preliminary insight to possible future prospective studies of this subject. Microbiome composition was estimated by 16S rRNA gene sequencing of 55 stool samples collected during approximately 3.5-13 months long periods. Detailed characterisation of the E. coli population was performed using colony PCR to detect 33 E. coli genetic determinants. In all children, genetic determinants typical for the probiotic E. coli A0 34/86 strain were detected immediately after administration of the probiotics. Analysis of the initial sample composition (the last sample taken before the probiotic administration) showed that the gut microbiome of infant and toddler with lower bacterial diversity was more successfully colonised by the probiotic strain. In our case report of three children, we showed for the first that supplementation with CNB probiotics in early infancy and toddlerhood was associated with high E. coli A0 34/86 colonisation and a significant change in the composition of the gut microbiome. Our results indicate that administration of CNB for its recommended duration might be efficient only in very early childhood.