Waterproof, but not warm enough

I have these boots ordered directly for my daughter as she gets slightly cold feet, I was looking for suitable boots and according to me. The product reviews of these boots seemed suitable because it down to minus 20 degrees Celsius stops lt. Description. In Gatschwetter he is tight, that's true, but now in the snow not warm enough. I tried it with a thermal liner and a second pair of socks, but unfortunately without success. Outside after the sledding and New Year's Eve in the snow had our daughter always ice-cold feet, so the boots go sadly back!