You can only restrict some applications that come with the phone. There is no way to restrict instagram and snapshat specifically while allowing a third party alarm clock app or organizer app.
??? Sure you can, and existing products already do something roughly similar...
Amazon's FreeTime works on a whitelist basis. As a parent, I specifically chose to allow which of the installed apps my children can use.
Amazon's FreeTime Unlimited, a bit like Amazon Prime Video, provides access to 9000+ videos, apps and books. They are curated and assigned an age-appropriate level. As a parent, I can specifically chose to let only material appropriate for 2-4 year olds be available on my Kindle Fire. The curation also indicates whether a given item is education or entertainment. As a parent I can have the software allow 30 minutes entertainment a day and that only after 30 minutes education. It also curates youtube videos, so as a parent you can let your child browse just an Amazon-curated subset of youtube.
Disney's Circle works at the wifi level via ARP spoofing (for folks whose routers aren't as configurable or friendly to configure). It lets you specify, for instance, which social media sites are allowed to be served to which MAC address and which aren't, or at what times of day. Yes it can certainly limit snapchat and instagram without limiting BBC News.
Apple's Guided Access sort of works with i-devices in more limited ways, but kids by their early teenage years pretty soon learn to disable it by powering off the device and then powering it back on again. And then social-engineering the passcode out of their parents.
That was just the technical answer to your comment. On a parenting philosophy level, I'm following an approach called "RIE" promoted by Janet Lansbury. It stresses that it's important to respect your children (e.g. give them freedom in their playtime to make their own autonomous choices), but always give them the solidity of knowing there are boundaries (e.g. it's in their nature to ask for more than what they should have, and it ultimately reassures them when they can't get it). I've so far found that a whitelist like I use with Amazon's Kindle works best for this, in hand with me setting time limits. It's certainly better than hovering over their shoulder all the time -- that would take away their autonomy at a time when they should be developing it, and I'm not the helicopter parent either at the playground or at home.
On another parenting philosophy note, after reading stuff about "The Paradox of Choice", I believe it's bad parenting to give kids huge unlimited choice. So they get the autonomous choice to use their device for only 4 things that I think are age-appropriate and formative right now... at the moment, Sesame Street, Mr Rogers, videos of their grandparents from the other side of the world reading stories to them, and The Snowman (soon to be replaced by Kipper The Dog). And one game, "Peekaboo Barn".