Kettles are as expensive as you want them to be - mine cost $14.99 in Walmart a few years ago. With tax added, probably about $16. You can get more expensive ones of course.
You can get electricals in Target, Walmart, Bed, Bath & Beyond, and many other places. B,B&B sends big coupons in the mail that you can redeem in store or online.
Toblerone is everywhere, and cheap, Tetley 'British Blend' tea is in Walmart along with Twinings, many supermarkets carry Irish tea brands, also many varieties of Twinings, McVities choc digestives are in major city/well-off suburban supermarkets.
Bovril can be hard to find, but you can buy Marmite.
You can get the syringes for infants here too; they just don't come in the package. The pharmacist will give you one if you have a prescription.
Some formulations of Infants or Children's Tylenol or Motrin come with a little syringe in the packet. You tip the bottle up, insert the syringe in a hole in the top of the bottle, and fill accurately. Very handy. Some just come with the little marked plastic cup style dispenser.
I would stock up on codeine products (check expiration dates), especially if you do any form of strenuous exercise or have a bad back, etc.
For school supplies, you will get a list as long as your arm for private schools, and a not so long one for public. Get the items on the list. Don't wing it. Private schools may also ask for room supplies like paper towels, plastic bags, antibac wipes, boxes of tissues. Your DCs will go to school weighed down like pack animals on the first day 
.
All school supplies are available in Walmart and Target, but after the end of August you pay more and some shops only carry a limited selection into the Autumn. Best to shop early. If you arrive in the Autumn, get supplies delivered via Amazon.
The T-I calculators can be bought on eBay and elsewhere online. We've only ever bought reconditioned and they've worked fine.
If your DCs are going to private schools they will most likely wear a uniform. Best to identify the supplier and order asap. You usually can't buy specific uniform items for private schools in shops, with the possible exception of non-logo trousers, polo shirts, socks of a specific colour. You may find that private schools have a uniform bank. If you're going to public school, don't go overboard buying top of the line clothes. American children wear very ordinary clothes and while fashion is important it isn't the huge preoccupation it tends to be in cultures where school uniform is worn.
You can get great bread in nicer supermarkets in the US, especially in larger cities. Best to avoid the sliced bread section.
Any Whole Foods Market will have nice bread, incl sourdough, and you can experiment with your local supermarket's bakery section.
You can also find decent breakfast cereals if you're prepared to read ingredients.
YY, if you're going to get American beds, leave your British linens behind. They won't fit. Bring your duvets though.
Wean your DCs off squash. You won't find it here.