Reddit mentions of Silicon Power U3 128GB Micro SD Card Nintendo-Switch Compatible, SDXC microsdxc High Speed Class 10 MicroSD Memory Card with Adapter

As others have pointed out, many games on the Nintendo Switch allow you to play with just one single JoyCon (one "half" a controller). It's not the best way to play games generally, but it works, and it's going to be the most versatile controller you can get, so you may just want to get one more set of Joycons for a total of two pairs. The Pro Controllers are really nice, and they are my preferred way to play games (I think they just feel better), but they're not necessary.

Mario Party is a decent game, but many complained that there just wasn't a whole lot of content in this version of the game. It's not bad, but it may be a bit disappointing.

The absolute best game you can get for a couple of youngsters for the Switch is going to be Mario Kart 8 Deluxe. It's an excellent game that looks great and plays great, is straightforward and easy to control, and has pretty wide appeal. Honestly, this is one game I think very nearly any Switch owner will like.

Captain Toad Treasure Tracker is going to be another good one for your girls. Not only is it a good game that's age -appropriate for your girls, but it has a co-op mode as well.

I also recommend World of Goo, a game that has you building towers and bridges to get a bunch of goo balls to an exit pipe.

I also recommend getting a good MicroSDXC card so you have plenty of space to download digital games, as there are quite a few great games on the Switch you can only get on the eShop. It looks like right now you can get a 128GB card for $15, a 256GB card for $28, and a 512GB card for $70. You don't have to buy this brand, but you should definitely get a card that's either made by a reputable company or one with a decent number of good reviews, as some manufacturers sell garbage cards. To give you an idea, games on the eShop range in space, but an average game will take up anywhere from a half a gig (or less) to as much as 32GB in some cases. And be aware that even if you just buy physical games, you'll still want storage space, as some games sold in stores only contain a download code.

Go ahead and head over to Nintendo.com and set up an account so you can start getting digital games as they go on sale - especially with Black Friday around the corner, expect to see some decent sales coming up. There are a few games on sale right now that you'll probably want to get, since they're decent titles and age-appropriate for your kids: Don't Die, Mr. Robot is an arcadey-style game where you're moving around trying to avoid enemies. Ninjin: Clash of Carrots is a cutesy arcade brawler-style game (like the Ninja Turtles games or Double Dragon) with some good co-op action. And Car Quest is an interesting game where you're driving around a fantasy realm and unlocking more of it as you explore. All of these are on sale for under $1.50, so it's not like you're taking much of a chance on any of them. You can see all the games currently on sale on this page:

https://www.nintendo.com/games/game-guide/#filter/:q=&dFR[generalFilters][0]=Deals&dFR[platform][0]=Nintendo%20Switch&indexName=noa_aem_game_en_us_release_des

(hopefully that link works... not sure if Reddit will take issue with all the punctuation in it)

Oh, also, especially since this thing is going to be handled by young kids, you want to get a tempered glass protective screen for the thing, and maybe a case too. I'm sure the last thing you want is for one of the girls to drop the thing and break its screen. Oh, and if you want to have a set up in multiple rooms, you'll want an extra dock. As much as you might be tempted to get a cheaper one, you'll want to get a dock that Nintendo makes - third-party docks have a bad reputation, and some have been known to damage the Switch.

Hope this helps!