What is the difference difference between this and the black and silver junior model?

Hello and thanks for your question. The most important difference is the t-bar width slider, which helps with getting the correct width. Then, the UltraFit is made of Lexan, a high-quality plastic, which allows for a more colorful device and lower price. All other Brannock devices are made of metal. We can also recommend this newest version of the junior device, which has both the t-bar slider and is made of metal. https://www.amazon.com/Brannock-Devices-TbarJr-Junior-Device/dp/B01A95PHYY Please feel free to email us if you have any more questions. All the best, BrannockDevices

My kids are toddler 6 and 7, is this too little for them?

No it's not too little for them. I bought this to measure my granddaughters feet , at the time one was a size kids 12 and the other a kids 1 and I was afraid it was going to be either too small or too big. they are now sizes 1 and 2 1/2.

What is the largest size this measures? My kids are currently 6.5 youth and 7 youth - both nearly at the end of kids sizes!

Hello and thanks for your question. The Ultra Fit Jr Brannock device measures from infant size (US) 0 to childs (US) size 5. After that, please see the Combination (or Mens or Womens) Brannock devices: https://www.amazon.com/dp/B075F121KB?ref=myi_title_dp Be sure you are getting a Genuine Brannock, not a knockoff or old "used" one as these are not guaranteed to measure correctly. Please email us if you have more questions and we'll be happy to help. All the best BrannockDevices