Not Sure About Sizing

I was a little confused on the size as I purchased 6.5 Big Kids and there isn't a lot of information about it. My grandson that is 3 wears a size 8, I wasn't sure what size to get for his little brother who is 2, so I thought perhaps the 6.5 would fit. However, these are closer, and a little bit big, for the 3 year old. My recommendation is to just go by the length in inch/centimeter, and seller should list all sizes with their length. Right now only little kid and toddler is listed.

These are cute, the colors are fun and bright and I know he is going to love the little car applique. They're not completely plastic, they have a softer feel to them so I don't think they'll be uncomfortable to wear. The little flap on the back is great for keeping them on, or if they just want slip on, it can be flipped up over the top. The bottoms have been created to allow for traction, with raised, ridged areas, although they haven't been tested out, this is a nice feature. 

Overall a cute set, just really need some help on the sizing.