If you don't want to have to know the sales tax for your location, I believe the highest sales tax in the US is under 11% so just give the kid $5.55.
Pretty much everywhere else I've travelled in the world, the price on the shelf is the price you pay, even when tax changes based on what something is classified as (to whit that fun legal case of "is a jaffa cake a cake or a biscuit").
For some absolutely bizarre self-hating reason, the US has decided that, no, your experience should be "Oh wait, now I've got to go and add some seemingly random sales tax that I can't likely predict in advance, and won't know until I've rung everything up".
It's just one of many little ways that the US operates that's profoundly at odds with a positive experience for citizens. You _could_ have the prices right there in the stores. You _could_ structure your tax system such that the prices can actually be done that way and you don't get the rounding fun that you see mentioned elsewhere in this thread. However, because you do this "hide the actual price" there's zero incentive to actually implement tax approaches that are nicer as a citizen/consumer.
If you'll forgive a slight diversion, it's a similar deal with tax incentives. In a lot of countries around the world, they structure it such that the tax incentives happen at time of purchase, so e.g. a $2,500 rebate would result in you, as the buyer, buying at price $2,500 less than it would have been otherwise. You never see that $2,500 at all, you just see a much cheaper sticker price. It's end user focused, like you'd think a government for the citizens, by the citizens, would be.
The US approaches it such that you have to actually have the money spare up front, to eventually get it back (or mess about with your monthly withholding to try to claw it back that way, quicker). This penalises poorer citizens, who otherwise might have been able to afford to buy whatever thing it is that they could potentially afford. You essentially miss out on the tax incentives the government comes up with, unless you happen to be rich enough to afford a well versed tax accountant, or have enough luxury of time to sit there and work through all the possible rebates you might be eligible for. Neither of which poor or disadvantaged people tend to have.
With this being the way that tax incentives have long been done, there's no real incentive to try to do it any other way, even though it would be better for literally every citizen, rich and poor alike, with the possible exception of tax accountants. You'd all get specific rebates you were entitled for.
You're overlooking the context. Someone said it was easy to let young children go shopping in places where taxes are included in the sticker price because you can give the kid $5 and the kid knows they can buy any item with a sticker price that does not exceed that or a combination of cheaper items whose sticker prices do not sum to more than 5$.
The implication was that this is harder for young children in the US, presumably because young children won't be able to do the complicated mathematical finagling that the rest of the world apparently thinks Americans do every time they go to the store.
My point is that in the US you just give the child $5 plus enough to cover tax, and then the child can use exactly the same approach those kids elsewhere use: if sum of sticker prices <= $5 they have enough money.
Cook county tax 2.75% Illinois state tax 6.25% Chicago city tax 1.25%
This thing I bought had a sticker price of $11.90
How much did I pay?