My opinion only, but this is my list.
Necessary:
Get together - Clubs and quite a few activities. You can make a cool teen hangout with the arcade machines. Clubs basically allow you to populate community lots and somewhat dictate the actions of nonplayables. For that it's worth it. I have some townies I make clubs for just so I can have my playables have more filled and interesting community lots. It also makes it easier to summon family members to do activities together.
Seasons - More activities and some nice things for toddlers + kids. Of course the weather, ability to plan events, and create holidays or celebrate premade ones. All very nice for family play.
Parenthood - Adds interesting game mechanics around parenting so your parenting will affect how your sims behave past teenhood. This makes some sims more sensitive or compassionate than others. A mean sim with the insensitive trait is far more nasty than just a regular mean sim. The board helps to manage families and set limits on teens and kids. So curfew and things like that. You can also punish and help in projects from school. It just makes everything about families more engaging and interactive.
Toddler Stuff - I think this is worth it just to get more things for toddlers to do and use. I use the slide a lot in my houses and community lots.
Kids room stuff - Adds basically a pokemon ripoff that kids can play which is kind of fun and cute. They can fight each other with their cards, collect them, and trade them (or gift them with seasons). Plus adds more CAS and Buy/Build items for kids and teens.
Nice to have depending on your style of family play:
City Living - Depends on how much you desire the world or some of the activities it includes. I really don't use any of the festivals.
Cats & Dogs - Depends on how much you desire the world or like pets. They do add a good bit of family play, but if you aren't into pets then it won't matter.
Jungle Adventure - Depends on if you'd like a vacation world. If so I'd get this one. A lovely location and there is tomb raiding for sims that are older. It can be very lucrative and kind of fun as a main career. Who can beat a jungle for a family vacation? JA definitely outshines outdoor retreat in this.
Vampires - Get this GP only if supernatural stuff interests you. I'm a family player, but I do love supernatural stuff. Vampire kiddos are so cute and I love how much more depth these ones have to previous vampires. If that doesn't interest you though then not something you need.
Laundry Day - This depends on how you play. This basically adds nothing gameplay wise except laundry. The CAS and Build/Buy items are wonderful though. I love them a lot, but if you are sure you'll never use or want laundry then it's not worth it to get it.
Backyard Stuff - Adds slip and slides and some other small items that I do think offer some gameplay value to my families.
Completely optional:
Spooky Stuff - A decent add on to basically make use of the seasons expack's ability to create holidays. I really never used the stuff until Seasons. So if you aren't buying that no use for this.
Dine Out - I don't think this adds family gameplay really. Toddlers don't work as well with restaurants so you can't really include them without a ton of hassle (it came out before toddlers). It does allow some family moments at fancy restaurants, but again it's all kind of badly implemented. The meals take forever. I usually only use it for dates now because anything more than 2-3 sims becomes unmanageable. A breakfast can last like 6 hours in sim time.
Cool Kitchen - Basically the only item that is included here that sort of helps family play is the ice cream maker, but imo not worth buying just for it. I'm not sure it adds that much family gameplay. If you don't get Dine out probably not worth it. If you do have Dine out then you can make a ice cream parlor which is pretty much all I use those packs for anyways lol.
Movie Hangout Stuff - I use this one more especially the CAS/Buy items. It allows for your families to watch movies together and popcorn is kind of a nice add on as well. Again though if movie night watching isn't your thing then no reason to get this. You are basically paying for the movies and popcorn items.
Vintage Glamour - I don't really think this is useful, but if you play a lot with a maid a butler might be interesting to you. I honestly never hire npcs so it's not worth it to me. There is also the makeup table for kids to play around with, but most of the items are for adults. Only a few interactions you will gain from kids interacting with adult objects. That's not really worth it to me.
Bowling Night Stuff - I consider this optional because without mods I don't find bowling works well. It often ends up being long and tough to manage with mods it's easier. Still it does add something enjoyable for families or teens (my bowling place is mostly a teen hangout with an arcade).