Don't download games unless they are on sale cheap! We have 2 switches with the same user profiles on each. You register one or the other as your primary console. Say son A has switch 1 as primary: he can purchase a download and anyone registered on switch 1 can play it, he can also play it on switch 2 but should son B attempt to play on switch 2 he can't. Son B has switch 2 as his primary: he can purchase a download and anyone registered on switch 2 can play it, he can also play it on switch 1 but should son A attempt to play on switch 1 he can't. In this case you need to buy 2 games and downloads cannot be sold on when they get bored.
We have loads of games but only have 2 copies of a couple as most can be played in multiple on a single console. It's only when they want to play the same game separately at the same time or together on separate consoles that you would need two copies and not all games are equipped to do this. Also to play together on different consoles some games use local play, others use online which you can only do if you pay for Nintendo online.
If you do this make sure you buy the family option (you may need to set up your own account). Single accounts on an adults profile can be upgraded to family at a later date, this cannot be done on a child's account (as I found out the hard way) and it is much more expensive to pay for Nintendo online separately.
Lastly shop around. If you do buy downloads you can buy codes from online retailers for both online games and Nintendo online to redeem on the online store which are often cheaper.