I gotta echo the concern, and I'm never too shy to speak up.

I trust the children all have proper PFDs for their age/weight.  Everyone wears a PFD, the only exceptions are people with death wishes.

Lakes are every bit as dangerous as rivers.  An overloaded canoe with an unstable load is recipe for disaster.

As a child we would have 4 of us in a 16' cedar/canvas.  Total weight ~600# and we sat still.  No we didn't wear our PFDs back then, they were for sitting on!  That was 30+ years ago.  Dad's in his 80s now and occasionally talks about the stupid risks we took back then.  A different time.

I don't know what you'll get for $500 brand new.  Maybe used, but I think what you need is over $1000 for the cheapest & heaviest model.

Kudos for planning to practice dump and rescue.  Please remember the first rule of rescue.  Save yourself first.