How common is urinary track infection in toddlers 1-1.5 yrs old? What are the causes?

Not very. Toddler girls are slightly prone to get utis as they approach toiet training age, <3%. Train them to urinate with thighs spread to reduce risk. Toddler boys uncommonly develop utis. Utis are more common in baby boys up to 6-months-old, and most occur in the uncircumcised (4 -10 x more)+ is related to foreskin. Utis in toddler boys often related to anatomical abnormalities + should be evaluated.
1% to 3% In this age range, up to 3% of girls and 1% of boys will have utis. The cause is bacterial growth in the urinary tract. Among boys in this age range, utis are more common amongst uncircumcised than circumcised boys.