We know Fred and George's shop is immensely popular with wizard kids. However, a large majority, about 97%, go to a boarding school year round. I could understand if they had a shop in Hogsmeade, but it is in Diagon Alley. I also know they have a delivery service, but is it really enough to keep them in business?
The twins have a line of Defense Against the Dark Arts products that provided quite a bit of business for them during the events of book 6:
"Giving him the tour? Come through the back, Harry, that's were we're making the real money....."We've just developed this more serious line", said Fred. "Funny how it happened...you wouldn't believe how many people, even people who work at the Ministry, can't do a decent Shield Charm", said George....
"The Ministry bought five hundred for all its support staff. And we're still getting massive orders. So we've expanded into a range of Shield Cloaks, Shield Gloves.."
As to the effectiveness, it is likely that the magic is fairly good, as Hermione proclaims about another of their (the twins) magical products:
"You know, said Hermione, looking up at Harry, "that really is extraordinary magic!"
Additionally, to the point about student orders, they have mail order and their Skiving Snackboxes, Love Potions, and other products are things that students may be more inclined to send away for than purchase with parents in tow.
- Harry Potter and the Half-Blood Prince, Chapter 6 - Draco's Detour
JK Rowling also stated in 2007 that after the war, Ron joined George at the shop and the two continued the success started by Fred and George:
J.K. Rowling: Ron joined George at Weasleys' Wizarding Wheezes, which became an enormous money-spinner.
J.K. Rowling: Well, I don't think that George would ever get over losing Fred, which makes me feel so sad. However, he names his first child and son Fred, and he goes on to have a very successful career, helped by good old Ron.
Fred and George get a lot of mail orders, not just in-store sales.
Fred and George sell a lot by mail order, in addition to what they sell in their shop. Their business was originally run entirely by mail order, and they’d been selling mail order products before they quit Hogwarts and decided to run their business full time. They only started selling in a store after Harry gave them his Triwizard winnings and they were able to find a place.
“Joke shop still on, then?’ Harry muttered, pretending to be adjusting the nozzle on his spray.
‘Well, we haven’t had a chance to get premises yet,’ said Fred, dropping his voice even lower as Mrs Weasley mopped her brow with her scarf before returning to the attack, ‘so we’re running it as a mail-order service at the moment. We put advertisements in the Daily Prophet last week.”
- Harry Potter and the Order of the Phoenix, Chapter 6 (The Noble and Most Ancient House of Black)
Fred and George still ran a mail order portion of Weasleys’ Wizard Wheezes even after they got their shop, and sent their products directly to Hogwarts when the students there ordered from them.
“But I thought all the owls were being searched? So how come these girls are able to bring love potions into school?’
‘Fred and George send them disguised as perfumes and cough potions,’ said Hermione. ‘It’s part of their Owl Order Service.”
- Harry Potter and the Half-Blood Prince, Chapter 15 (The Unbreakable Vow)
So, Hogwarts students would be able to buy from them year-round, and they did.
In addition, Hogwarts students aren’t their only customers.
Though Hogwarts students would be a large part of their customers, Fred and George also had others. There’s a group of ten-year-olds in their shop, who’d all be too young for Hogwarts.
“A space cleared in the crowd and Harry pushed his way towards the counter, where a gaggle of delighted ten-year-olds was watching a tiny little wooden man slowly ascending the steps to a real set of gallows, both perched on a box that read: Reusable Hangman – Spell It Or He’ll Swing!”
- Harry Potter and the Half-Blood Prince, Chapter 6 (Draco’s Detour)
In addition, since they’ve expanded to selling things other than joke products, they’d bring in even more types of customers. Their Wonder Witch products, like Ten-Second Pimple Vanisher, could appeal to witches who’ve left Hogwarts as well. Pygmy Puffs could appeal to anyone who wants a cute pet that’s easy to take care of. Their Defense Against the Dark Arts products would appeal to anyone who wants extra protection, including the Ministry.