Selecting the proper size and fit on shoes is super-important, especially for young feet that are still being formed. In these days of internet shopping, parents and other shoe buyers have to take on the role of DIY "professional shoe fitters." Feet come in a huge variety of lengths, widths and shapes. So do shoes. "Sizes" are only approximations and vary considerably among brands and styles and lasts (the shoe mould).  So the "size" of a current pair might be unreliable. 
Foot length - there are several ways to do this ---
1) Super easy is to slide a tape measure into a pair of shoes that fits well and compare that measurement to the measurements at the end of every listing. (Don't rely on the sole/outsole which is a fashion/style element and has little to do with insole/space available for the actual foot.) If the current shoe is tight, allow a maximum of 3/8" for babies and between 3/8" - 1/2" for bigger kids.
2) Rulers: It is also easy to have a child stand on a ruler.  Do this for both feet. For a non-walking baby, the measurement can be taken at any time, just be sure that the toes are stretched out and not curled. The measurement should be taken with the child wearing the legwear/socks that will be worn with the shoes. Then allow for growth/wiggle room. A maximum of 3/8" for babies, between 3/8" - 1/2" for bigger kids. For skimmers, 3/8" is about max.
3) Foot tracings: Again, this is a standing measurement and both feet should be done. Try to trace as vertically and as close to the foot as possible. Then it is easy to measure the foot length and continue just as in #2.
Foot shape is the other dimension and it is important to take width, instep height and general shape into consideration. This why shoes are manufactured on different lasts, the moulds that give shoes their shape. De Osu and Faro offer different lasts for different shaped feet.  So if one last/shape doesn't work for you, there is most likely another than would be perfect.  Shape information is always helpful.
We are always glad to help.  We even receive foot tracings by e-mail, which we cut out and "fit" in shoes.  If you do trace and e-mail, please write the foot length on your tracing since scanning can distort the measurements. A tracing that started out as 6 1/2" may arrive as 6 1/4" and definitely mess up the sizing system.

I'm certainly sorry that shoe fitting is not as simple as we all wish. But it is worth the effort to ensure that your child's feet will be comfortable and healthy.  As always, consultations perfectly OK.