It is a small entertainment experience prior to a normal retail shop. Ollivander will assist one (or two, in case of siblings) wizards select their wand. Not every child in the room, will receive special assistance, and the child (or sometimes adult) is picked randomly by Ollivander, so there is no way to ensure your child gets picked. The wand is NOT free, sometimes that causes issues from parents/kids who think because they were chosen, that they can keep the wand.
It recreates the wand choosing, from the book, where the wrong wand causes trouble and the right wand reveals itself. It's about 5-10 minutes, and it's a cute experience but not necessarily worth the wait time, when it was only available in Hogsmeade.
However, the Diagon Alley version has many, many rooms, so it's a very short wait, and everyone must view the show before entering the wand store. It still seems to me, that there are longer waits in the Hogsmeade store, so if you have a choice, I would just go to the one in DA. I
In the store, there are assistants to help you pick your wand, but there is no special show. But they can tell you whose wand is whose if you see one in the case you like. Or identify the wand that corresponds to your birthday month. All wands are about $45. There are many places in both lands where they trigger things to happen, and playing with them is a lot of fun, even for adults.