My son has a different surname to me and was on my passport ,
If I remember right....you needed permission from other parent to have child on your passport...so fact child on it meant permission given.
I had my step son on my passport who didnt have my husband name and therefore also had a different name to mine...who though he stayed with us whenever not in school, didnt even officially live with us ....and my husband had never even been married to his mother....he only had parental responsibility.
My husband couldnt come with us so I was taking children on my own...and I felt it was unfair to leave him out...it was the first family holiday abroad....specially chosen to suit the children....going euro disney then on to Salou in Spain.....so when I applied for my passport he was put on after production of marriage cert , husbands proof of parental responsibilty, and letter of permission from his mother.
I also had another step son who had husbands name ...so same as mine....who lived permanantely with us....and my natural son ...who because I had remarried had a different name to mine...they were both on my passport as well.
I also took with me my natural son who being 15 had his own passport...thought it wasnt worth putting him on mine only to need to get him his own in a few yrs time....he had taken my new married name and for officcial reasons had it double barreled with name on his birth cert.
My step son who has same name to mine was the youngest....he had a little friend ...who was the child of a close friend of mine who also happened to live next door to me ....they were inseperable ...so her mum got her a passport and I took her too.
The only one I took a letter for was my friends little girl....giving me permission to take her ....and also giving me permission to act in parentus locco ....signed by both parents.
The fact the others were on my passport meant permissions already given....and eldest of an age to answer for himself.....but shudder to think of the hassel I would have these days trying to do same thing