What is recommended dose of peg 3350 (to help against constipation) for a 6 years old 18 kg girl. it comes in packets of 17 g each but that packet i believe is an adult dose?

Varies greatly: There isn't a right or wrong dose for this particular medication. The right dose is the one that works to soften stools without causing Frank diarrhea. I would start with half a pack in 8 ounces of water or juice. Then every 3-4 days, adjust dose up or down depending on results. If still having hard stools, increase to 3/4 packet in 8 ounces daily. You can go as high as 2 packets daily.