赤ちゃんが乗っていますって英語でなんて言うの？

The most common expression is:
'Baby on board'
It is a sign often used in the U.K to show that a baby is inside the car.
In terms of the statement, you can also so
Baby in car.

I hope that helps!

You can use all of the above to express to other drivers that you have a baby or kids in the car!

"On board" or "aboard" meaning in the car

"baby on board", "on board" means on or in something that can fit a person. When you say baby on board, it's another way of saying there is a baby in here.
"there is a baby in the car" is a direct way of saying what it is, that there is a baby in the car.
"careful, baby inside" signifies to other drivers to drive carefully and watch where they're going because there is a baby in the car.

Yes, your ideas are the normal signs or messages that there is a baby in the car and these are possible messages. It may not be the best idea to write anything on a sign yourself,,probably better to buy a ready made sign with clear letters and clear message as drivers will drive closer to you to read the message if it is not perfectly clear. A popular message in the UK is 'Keep your distance' which means do not drive too close to the back of your car. It may be safer just to have a diagram or pictorial image of a baby so people are not distracted by reading messages while driving behind you.

In the uk it is common for people to have signs in there back window usually to say "baby on board"
This allows other road users to know that there is a baby in the car and to be careful around them
You could also say "There is a baby in the car."

A sign usually used to indicate that there is a baby in the car, then you can say: Baby on board.

There are usually ready made signs that say this that are sold in stores.