No one said .049 size lines.  A low powered Baby Bee would likely go .008" to .010"  @30' to 35'.  If you have a Tee Dee, Norvel or Fora, 42' would be standard.  A pull test is always recommended before each day's flying or after each crash.(if that would ever happen)  There is some stainless steel fishing line that seems good for 1/2 A.  I like a .09 to .15 motors myself for economy and more importantly for kids, they won't get away on them if the lines go slack and pull the handle out of their hands.  A thong is a good idea.  I had a sore shoulder for a couple of days when a .40 plane recovered from slack lines and I am a fairly big guy. It didn't hurt at the time though.  .049 motors are not too good in the wind and are kind of finicky sometimes unless you get a Fora or something like that.