Our DCs were in US state system for 3 years, one went into 1st Grade and the other into 3rd as although they were technically 'a year ahead' we wanted them to be with kids of the same age. The curriculum tended to be pretty similar in maths and English (apart from spelling!) And they did lots of fab creative stuff and PE. Non uniform was great, lunches not great so we sent packed lunches. Depends on the school district, but the teachers at ours were fantastic.
They both did well at school and as PP said, it gave them a gentle first year to settle. We've been back in the UK for a few years now, both at high school, and both excelling so the US experience didn't hinder their academic progress. On the contrary, the experiences we had and the places we saw as a family were massively beneficial.
Things to be aware of: the school day tends to be longer than UK (8am-3.30pm) with fewer breaks, often only one at lunch; the semesters are long and most schools close for 12 weeks over summer...that's a LONG time off!
They'll likely need extra vaccinations than standard UK ones to be eligible to enter state school system.
They do routine fire, tornado (southern states) and intruder drills which can be unsettling coming from the UK.
If you're coming back to the UK, br sure to teach DCs the metric measurement system as US is still imperial.
Remind them to ask for an eraser, not a rubber, in class 
Good luck with the move!