I found dry suits for that size are very hard to find.  By the time my kids were comfortable going down a river I bought the Kokatat kids suit but it is sized for a 14 or 15 year old.  We bulled the suit up till the crotch was in the right place, then folded the body and held it in place with a cam strap that had the extra length cut off.  The legs were baggy, but not too bad, same with the arms.  Worked for us during the early years.