Louvre

In essence, there are two lines: one to buy tickets and one for security. So by booking ahead, you skip the ticket line, which can be long, although there are several locations to buy tickets and several entrances. The kids under 18 don't need tickets and just walk in with you. If anyone asks for proof of age (if they're close to 18), then passports will provide that.

This note is from the shopping cart:

The ticket purchased online is time-stamped and nominative, an identity document may be demanded.
It is valid only for the chosen service, date and time. It is not a queue jump but gives access to the pyramid within half an hour of the time shown on the ticket. If the holder of the print@home ticket does not arrive within the allocated time slot for admission to the pyramid, they are subject to the same admission and waiting conditions as visitors without tickets.

The underground mall entrance is usually less crowded than the pyramid entrance.

There is a separate line for those who have passes or tickets. If the line it is long and it looks like you won't make yoiur window, go to the head of the line with that time stamped ticket for advice; they may let you in right away. We have never had a long wait at the Carrousel but the timed ticket has no priority for security there.

I don't understand the question about how you print tickets. You buy the tickets, they send a link and you print them like any other tickets. The kids don't get or need tickets. They just walk in with you. Make sure older teens have age ID -- passport, copy of passport (usually, not always, works), passport card. It is fairly rare for kids to be checked for age with their parents.

Tickets at the Louvre are sold in a large room with many machines off the courtyard under the Pyramid. There are not separate ticket sales locations for different entrances; the entrances are about security not tickets.

We were there in May as well and when we went to the Louvre we had timed tickets for the Vermeer and walked into the Carrousel entrance with no line at all and there were no lines for the ticket area if we had to go there. There are LOTS of machines. There is a good cloakroom with self service lockers to stow coats and small bags and motorcycle helmets. There is even a locking device to stow umbrellas.

When we were there in October, the line at the Carrousel for security was about 5 minutes and we got a ticket machine with no wait at all.

So you may be lucky on any given day or time especially before the worst of it in June and July. The Louvre is set up to handle a lot of people since there renovations of the Pyramid court.

Note also you can come and go all day on the ticket if you want to head out for lunch or a stroll.

I was there last September (didn't go when I was there in May). I had a museum pass. They actually had three places to line up. One for people who needed tickets, one for museum passes and another for prepurchased (online) tickets. I got there early. At opening there was a very short line for both pass and online tickets. The line for people needing tickets was probably better than 30 minutes (30 minutes before it was almost nothing).

The three lines fed into different security lines. I didn't have a pack and was simply waived through.

You can save time by having a ticket, getting there early so you're in the front of the line and limiting what you are carrying so you don't spend a lot of time in security.

I didn't have an umbrella that day but later, at Versailles, they had a rule that portable umbrellas were okay to carry with you but normal size umbrellas need to be checked. If it's raining, take the fold up. It might save you some time getting in.