That's actually a good thing. IMHO, with the way kids are trafficked now of days. Get asked a lot why I changed my name a couple times, and one was to be the same as my village orphan. Actually got her MIA parents in the same room one time, to get her PP as soon as I could. I think she was 15, and tested it out shortly after by hopping over to Laos. No issues at all since we had the same last names in our PP. Same with hotels and her govt ID. Farang checking into hotel, by self with Thai child should raise some eyebrows, if not having the same last name. Now she adopted us, when an adult, and we all have my wife's last name. Strange the rule isn't in place for up to 20 yrs old, as still a minor.