Toph,
Nothing wrong with Polygon bikes for the kids (or for the adults).
You can be pretty sure that any bike brand that can win Redbull Rampage can build bikes to handle whatever your kids are throwing at it.
www.pinkbike.com/news/kurt-sorges-polygon-red-bull-rampage-2018.htmlPolygon have their own factories and only sell direct via the web so no retail outlets is probably the reason they might look cheap. Componentry is pretty well the same as any other bike of similar range / value.
Could have a big argument about supporting your local bike shop but that's another issue.
Kids learn quick, but I'd say better to keep them sized to the right bike than to try to save some dollars and buy a bike that is too big hoping they will grow into it in the years to come. The right size bike and they will want to ride. Too big and they won't. I reckon the size of bike for them is way more important than having the latest 2021 Shimano XXX dooby whatsit.
Start 'em on little tabletops (yesterday) :
and move on to big gaps today :


- that's Norco in a 24" wheel by the way not a Polygon, but he had a Polygon in 20" wheels.
oh yeah, also don't let them watch Matt Jones videos on youtube (who rides a Marin, another direct to customer bike brand) or you will be building a dirt jump track in your back yard.
But most of all make sure you have a good story - like that you have to take the photos so you would send the gap, but unfortunately can't because I have to take the pictures, but could of I wanted to, really, I could.