Sure. I'm not even that sure if an international adoption requires the I-551 status at all. Legally if one is a minor child of a US citizen and physically present in the United States, they're automatically entitled to US citizenship. Doesn't matter if they're adopted by US citizens or the parents become naturalized.
And green cards are more or less green again.
This is one of the few topics in life where I am definitely an expert... through lots of annoying personal experience.
Kids can enter the US either having already been officially adopted in their birth country or on a visa where their parents are technically guardians "with intent to adopt" Which one is which is totally dependent on the birth country and the international treaties between them and the US and their status as a member of the UN Hague Convention.
If a child is officially adopted in the birth country, and that country is one where the US recognizes the adoption, then you're right - they enter the country as a child of US citizen parents and once they land on terra ferma, they are citizens. Assuming the people at immigration at the landing airport do their job correctly, the kid will automatically get a citizen SSN card (which is a 50/50 assumption, from what I hear), are eligible for a passport right away, etc.
However... if a kid is coming from a country where the adoption isn't finalized, or if only one parent travels to the birth country, then the kid lands as a permanent resident, because the parents are - legally - guardians and not parents at the time. Once the adoption is finalized in the US (typically after 6 months - a year) then the child becomes a citizen. In those cases, the SSA is a HUGE pain in the butt. They won't accept the adoption decree as proof of citizenship (even though they do accept US birth certificates as proof) In those cases you need to get some other paperwork - typically a certificate of citizenship at the cost of nearly $1000 and months or even over a year's waiting time - and go back to the SSA and prove you are a citizen and get it changed. Have I mentioned it's a huge pain in the butt? ;-)
Phew. Thanks for reading if you've gotten this far. It was sort of cathartic to write that all out.