IXL is an excellent tool for improving math skills. The kids are ALWAYS welcome to work on any skill they wish to! Skills can be repeated as well, meaning if they already did A7, rounding, and later they want to practice rounding again, they can go back and re-do A7. They can also go back and do 4th, 3rd grade work as review practice. The more the better!
When I assign homework, which I will often do, the rule is 15-20 minutes or master. If they have trouble with a specific area, and are getting frustrated, the can stop after making a good attempt at 15 minutes. Please encourage them to READ the explanation of WHY they got something wrong, if and when they do answer incorrectly. It is like having a teacher there, it tells them exactly where they went wrong.
They can go ahead, behind, anything they want at any grade level. I will NOT assign every section listed on 5th grade IXL, likely less than half of them. There will be other forms of HW as well, making it impossible to conquer every section. However if your child is ambitious and wants to accomplish them on their own, you should definitely encourage it.