Toddler tops generally run from around 1-2 years through to 3-4 years, sized in shorter increments because growth is faster and more variable at that age. Kids tops typically pick up from around age 3-4 and run through to 8-9 years, sized in broader, more standardised year-based brackets like 3-4, 4-5, 5-6 and so on.
The cut also shifts. Toddler tops are shaped for a rounder toddler build, often with more room through the tummy, while kids tops start to reflect a longer, leaner child shape as kids grow into a more typical school-age proportion.
Because the transition between the two ranges is not always a clean cutoff, checking the specific size guide rather than assuming a straight switch at a particular birthday is the most reliable way to land on the right fit, especially for kids on the taller or smaller side for their age.
If your child is right on the boundary between ranges, sizing into the smaller end of kids tops rather than the largest toddler size is often the better long-term choice, since it gives more room to grow before the next size change.