I need a replacement wheel for my kid's 24" mountain bike. I want to keep his front tire the same width 2.25", and just need to know the right wheel rim dimensions. I was referencing a wheel chart from WTB to make sure I'd be buying a wheel with the right rim width for a 2.25" tire, but the chart is a little confusing in that size as it seems to condense all 2.1-2.3" tires down the same rim range. So, according to this chart, I should get any wheel with a rim ID between 20mm and 25mm, right?
I completely agree that the WTB chart for mountain bikes is confusing. This is because the tire section widths in inches and the tire section widths in mm are inconsistent: the Medium category of 2.1"-2.3" should actually extend up to 58mm whereas it appears to only include 54mm. Given the inconsistency, I'd be inclined to go with the mm values since it seems to be these that determine the recommendations.
A 24" by 2.25" tire corresponds to ETRTO size 57-507 (see e.g. here, How many tire sizes are there?). And, according to the WTB chart, 57mm implies a suggested optimal rim width range of 25mm-35mm. The chart by Schwalbe available here suggests a wider range of 19mm-40mm.
Based on the WTB tire and rim fit chart, you are correct that for a 24" tire with a 2.25" width, the optimal rim inner width range is 20-25mm.
Your tire size of 2.25" falls under the ETRTO size of 54-559. The first two digits 54 indicate a tire section width of 54mm, which converts to approximately 2.25 inches.
For a 54mm/2.25" tire, the chart shows optimal performance with rims that have an inner width of 20-25mm.
So when looking for a new 24" wheel, you'll want to find one with a rim inner width in that 20-25mm range to match well with your 2.25" tire.
Any 24" wheel with a rim ID measurement between 20-25mm should provide good fit and performance with your existing 2.25" mountain bike tire based on WTB's recommendations.
So in summary, yes, according to WTB's tire and rim fit guidance, a 24" wheel with a rim inner width of 20-25mm is what you'll want to get for optimal compatibility with a 2.25" tire. That range will give you good options.