This is in reference to Hadith no. 222.
Narrated Um Qais bint Mihsin:
I brought my young son, who had not started eating (ordinary food) to Allah’s Apostle who took him and made him sit in his lap. The child urinated on the garment of the Prophet, so he asked for water and poured it over the soiled (area) and did not wash it.
The urine of a child (boy only) is not impure till a specific age. During the first few months, when he does not take anything except milk, his urine may be left unwashed if it touches anything other than his own garments. Sprinkling water over the soiled place is enough, just like the Prophet (SAW) did. But as soon as he starts eating, his urine has to be washed even if it’s a big mess.