I have first-hand experience.
When we moved to the US, we put our boys up a year, from UK Y3 to US G3, and Y1 to G1. We did this because it was a 4 year assignment so wanted to be ready for moving back when the time came. Also, if my younger DS has gone with his age group, he'd have moved into part-time KG.
In retrospect, this was the wrong thing to do. They were not on par with their new classmates, but a good half-year behind. In addition, most boys in our school district were held back a year, so were actually 2 years older than our boys. DS1 was fine, but the gap for DS2 was large and we would have had him repeat 4th grade had we stayed.
We had no trouble returning to the UK. They were probably a bit ahead in English and Maths, going into Y7 and Y5 respectively. They hadn't done any meaningful science, history or geography, but caught up quickly. They also started French in the UK, but with remedial classes were caught up by half-term.
DD1 completed KG in the US and went to Y2 in the UK. KG was just play, so DD started Y2 as a complete non-reader. She was on the top table by half-term.
My advice is don't rock the boat too much and don't worry. In the US, they start school later, but with more readiness.
I would be much more worried about the other end of compulsory education.