Single parents, grandparents or guardians traveling with children often need proof of custody or notarized letters from the other parent authorizing travel.
I'm a widow whose son was 4 when his dad died. Since then I have taken him to South Africa, America and Europe. The only "difficulty" I have had is in getting his passport in that I have had to swear an additional affidavit that I am the sole guardian.
A couple of years ago a friend took him to the Paris Air Show. I brought him to London and the friend took him on the LHR-CDG leg the following day. My friend is Chinese, my son is not! I provided them with a letter authorising the trip - even though it was a day trip - but no request was made of them