You might be able to place your 7yo and 5yo together in Level B (if the 5yo is ready – some are for level B, some aren’t) I imagine your 9yo would be in Level C, D, or even E… (make sure you get the transition lessons!)
When combining kids, you do need to be prepared to seperate them if it becomes necessary… (but you can worry about that if it happens….)
And yes – definitely have one of the older ones play games with them. The more games the better! Use the games book.