When we make a garment for a child we often think we should make it a few sizes larger to allow for growing room. This increases the amount of positive ease the garment is worn with. So if the child has a 20" chest, and we pick a size that has a finished chest measurement of 24", it will have 4" of positive ease.
Have you have thought about how the ease changes the look of the finished item though? Let's take a look at Pease Poridge.
Here it is styled on my oldest son:
The garment as styled is an open front cardigan. You can see it fits well. The sleeve and body are just the right length and there is no pull on the button. The neckline is close fitting. It is being worn with about 2" of positive ease. The sample is 3-4 years and Leo was 3.5 at the time (wearing a mix of 3-4 year and 4-5 year size clothing from the shops). As it was a magazine sample, it was knit to fit with no growing room.
And here is the same sample on Rowan. The garment now is styled as a jacket. You can see the sleeves are just a bit long and the body goes down onto his hips. The fronts completely touch and the neckline is much wider. It is being worn with about 4" positive ease. It's still the 3-4 year sample but now on Rowan who is nearly 3 (and wearing mostly 2-3 year size clothing from the shops).
So, same garment, two different kids, two different looks! Make sure then that if you decide to size up you consider how it will change to the look of the garment.