Given the range of permissible widths by the tire manufacturers, there's not a "one size fits all" answer to this question.
For example, Schwalbe tires up to 2.4, VEE up to 2.35 and Maxxis tires up to 2.5 WT will work. For the kids’ bikes Frechdax, Dreckspatz, Barney, and Bam Bam, it is recommended to use the factory-installed tire widths as a guideline.