One of my DS's friends was moved up a year in a state primary.
The child in question is incredibly bright.
The school were very vague at what the future held suggesting the other pupils might catch up if this boy hit a plateau. But he didn't and ended up with the best marks in Year 6, despite being a year younger.
Secondary transfer was a nightmare as it had to be done by special arrangement.
The secondary viewed it that my DS's friend was depriving another child of a place in an already oversubscribed school. They were not helpful as it is an unusual situation in state schools.
In the end the new head teacher of the primary was able to "blame" his predecessor for moving the child up in the first place and come to a special arrangement.
OP - Don't let them move your child without speaking to possible secondary schools about the effects of this in year 6. Our old head was keen to move the clever boy as there were too many in that class and it was convenient for him.
Also I would say that this friend is struggling socially because he has actually been deprived of appropriate peers by moving up. His social development HAS been affected and his Mum did suffer some nasty comments from other parents who did not see why their DCs couldn't move up too.
Its very flattering for the school to request this for your child but in the state sector I don't think its a decision that should be taken lightly.
Good luck.