Believe it or not, what's in the curriculum is actually really difficult and not age-appropriate in any way for the majority of children. It is also quite knowledge-focussed for the foundation subjects. This, combined with mastery teaching (an emphasis on ensuring that ALL children get it and don't get left behind), means that teachers are essentially forced into a way of teaching that goes really slowly through the content so that all the children are picking it up and mastering it really well. There are ways to extend more able pupils within the same content but frankly, I often feel so exhaused by the constant, unreleting focus on special needs and the ones that aren't getting it that I sometimes don't have a lot left for those at the top.
This. It's quite hard to really extend able kids if they already quite frankly, know the content inside out and you are not allowed to go beyond it - my DC is suffering with this. He's miles ahead in maths & reading (not in other areas mind!!) and is bored and it honestly makes him lazy.
Add to this that the government wants levelling. They do not want the gap between higher & lower attainers to be as big as it is. In reality, as more able learners often progress faster/more easily than others, only way to reduce attainment gaps means holding them back to allow slower learners to not slip further behind.
Few state schools really do an excellent job of meeting the needs of the more able, the reality is these kids learn like sponges & do well regardless, as long as their parents do plenty at home to stimulate them/provide challenge.
Op - can you afford things like music lessons to challenge them? Play lots of board games at home, look for a chess club or language lessons, computer coding clubs are becoming really popular. This is what my parents did for me & kept me mentally stimulated. I'm doing the same with DS. The music lessons in particular really help.