Undergarment for children comprising a babygrow (2) with fittability 0-3 years of age; in said babygrow (2) a neck (21)-crotch (22) directional line is defined; more than 90% of the weight of the fabric of the babygrow (2) is in cotton. At least 70% of the outer surface of the babygrow (2) has ribs (7) extending transversely to the neck-crotch directional line (3). The ribs (7) permit an elongation of more than 40% along the neck-crotch directional line (3) following the application of a force that is less than or equal to 5 Newton along said directional line (3).