If your grandkids like Nintendo first party games, the Switch is definitely worth picking up, IMO. I have put in well over 100+ hours into each of Breath of the Wild, Super Mario Odyssey and Splatoon 2. All three of these are great, IMO. There are the Wii U ports of Mario Kart 8, Captain Toad and Donkey Kong Country which are all fun games. The Wii U ports annoy me because I already have them on the Wii U. Not many people owned a Wii U, so I understand why Nintendo ported them over to the Switch. Super Smash Bros Ultimate is also coming out later this year.
My kids 7 and 10 mainly use the Switch to play Fortnite together. Since you can play cross platform on the Xbox One and Switch, they can each play on separate TVs. They occasionally play Rocket League cross platform, also.
The big benefit to the Switch is being able to take it out of the dock and use it in handheld or tabletop mode. I didn’t think my Switch would get used that often out of the dock, but my son uses it in handheld mode quite often. My wife also prefers to play Stardew Valley in handheld mode.
The downside to the Switch is power. It is not going to compete with the PS4 or Xbox One as far as power goes. Because of this your not going to get all the 3rd party games that these other two consoles get. This isn’t a factor for me because I buy Nintendo consoles for Nintendo games. We have an Xbox One for all the Call of Duty and other games my kids might want to play.
Later this month, Nintendo’s online service will be needed to play online. I believe the cost is $20 a year for a single person or $40 a year for a family. I could be a little off on the price. You will be able to download a bunch of older Nintendo games as long as you keep the online service active.