I agree that numicon could be very useful for your 6 year old and possibly your nine year old.
Some ideas are odd and even numbers, making number bonds to 10,20 first addition then take away thus showing subtraction is the inverse of addition. If you have a balance scale put 15+5 one side of a balance scale and say 17+3 the other side.
Also doubles (3+3=6 2 three plates match the six plate) and halves.
Halving can often be tricky to understand as some children memorize their doubles and need visuals such as numicon to see the relationship between the two.
Try counting in tens,fives etc which leads on to repeated addition 5+5+5+5, use the numicon plates to solve written problems,place value making numbers using the ten plates and other plates for units. Young children often confuse teen numbers such as 13 and 30 and having a visual such as numicon can help give a child a clear picture in their head what a number really means.
You can also use numicon to compare numbers . Look for image of greater than less than crocodile online.
There are many youtube videos showing how to use use numicon.
I work as a ta in key stage 1 and also tutor an older child who finds maths challenging. At work I have used numicon to support even the most able children.
If you see numicon in a similar way to cuisenaire rods and give the 10 plate a value of 1 then you could ask a more able child what is the red plate etc is now worth (decimals).
See cumbria grid for learning 100 things to do with numicon
numeracy.cumbriagridforlearning.org.uk/index.php?category_id=55