My young nephew many years ago, was about this age when he would yell the word “TRUCK!” and point at a truck every time we passed one on the road. But he had not yet learned how to produce the “tr-” sound so he replaced it with an “f-”.
His mother would calmly reply “That’s right, Sean. ‘Trrrrr-uck’.” And he eventually outgrew it.
LOL!! We went through that in our house! It was reallly funny when it was a dump truck, and he would yell it out the front window in earshot of the busy sidewalk....