If you are an experienced mitten-knitter and can read charts, here is a pattern for warm Lopi mittens for your 6-8 yrs old :-)

The chart shows left hand mitten, for right hand mitten place the thumb over first 5 sts - otherwise the same.

The size is easy to adjust for smaller or larger mittens (for an experienced knitter). For 4-6 yrs old: cast on 20-22 sts, for 8-10 yrs old cast on 26-28 sts. Try the mittens on to figure out where the thumb should be placed, and how long the mittens should be.