It’s confusing because you’ve got range dosing and there is no set ratio of liquid:powder – the mfg says mix 17g (standard adult dose) in 4-8 oz.
So here are my specific recommendations:

Anyone over 15kg gets 17g. Rx: Miralax 17g daily. Mix one capful or packet in 4-8oz fluid (For an adult or bigger kid, mix in 8oz. For a small kid, mix in 4oz.)
10-15kg: 8.5g po daily. Dissolve one-half capful in 2-4oz fluid (again you pick based on the size/age of the kid)
5-10kg: 4.2g po daily. Dissolve one-half capful in 4oz fluid. Give half the dissolved solution (2oz) and discard remainder.

Thanks to Erin Robey