I am a single mother, however my own mother helps out with my daughter and so I am no longer sure if I even still count as one.
Yet, I might as well drop in my 2 cents. I believe the biggest challenge is to provide your child with all of his or her basic needs such as clothes, food, shoes, toys, etc., and still be able to have some left for their school fees, and other monthly or daily expenses.
It really is a challenge for me, to be honest, and everyday I do my best to always try to make more so I could get more at the end of the month so I could have some left for the other bills (which are always behind).
So, in other words, the biggest challenge for a single parent like myself is money.