In general, the same rules mentioned in the post How Much Space Should the Toe Box Have apply to trying on children's shoes as for adults. But you need to keep in mind that children's feet grow quickly. And if you do not plan to change shoes right in the middle of the season, you can take a reserve of about 15 mm.

Moreover, according to Dr. Wieland Kintz (the same one who conducted research on the effect of shoe length on health), it is rather desirable for children to take something with a slightly larger gap on their feet. Their feet are softer, more flexible and they may need more space for proper movements./

☝️ But there should not be a reserve of less than 10 mm in shoes - this is too little for both children's and adult toes.

❗️ Important nuance: if your baby's shoes have the "correct" length reserve, but he stumbles in them - do not change them to shorter ones. Most likely, the shoes are simply too wide or do not fit well. It is advisable to look for another, suitable model, rather than compensate with a smaller size.