I purchased Minecraft (Bedrock Edition) so that my kids and I could play together (cross-platform). I'm playing on pc they are on the switch. My question is if I purchase a behavior pack and start up a world with that pack can my kids join that world without having the pack owned? I'm trying to keep from having to purchase it 3 times?
Your kids don't need to buy your behavior packs again. Although sometimes the game will prompt them to download certain ones, NOT buy them.
In other words, if you already have purchased a pack and uploaded one or more to a world, then basically anyone that is allowed to join can use them so long as they download the packs (if needed).
Hope you have fun with your kids, and don't get blown up by a creeper! ;)