Always use two fingers to check! Place your index finger under the sole and your thumb on top of the toes to get the best tactile feedback. You can feel the position of the longest toe (usually the big toe). There should always be at least 3-4 mm of space in front of the toe. This is different for conventional shoes with stiff soles and unnatural foot shapes. With Lieblinge, it doesn't need to be more. If the toes are already more than 3-4 mm from the front, it's time for the next size.
Extremely important: The very soft substance of an infant's foot is not yet a "finished" bone, but super soft and therefore malleable. Permanently wearing shoes that are too small will definitely lead to deformities, often resulting in permanent misalignments and possibly shortened tendons. So please follow these guidelines.