Is that must to have baby seats for children in Europe?
We are a group of 2 families planning to visit Germany / Swiss / Nethers by road. We have plans for renting a car and driving across. We have 2 kids 11 and 6 years old. The other family has 2 yrs old kid. Is that a must to have a kids seat for the kids? If so, Is there an age limit for the kids to have baby seats?
Is this allowed to travel in a 5 seater car? (4 adults + 3 kids as said above).
Thanks in advance.

1st site below is for the U.K., 2nd site for other European countries.
Whether it is a must or not, why would any loving parent want to risk the life of their child by making them ride without the safety of a carseat?
As you can see in Charlies answer you need booster seats for the 6 year old, the 12 year old may go in a normal seat or also needs a booster seat, the 2 year old need a car-kid seat.
I think it will be hard to fit the 3 kids in the back of the car with the seats, you will certainly not be able to add two more people.
If you want to travel for any time with these 7 people in a car you need to rent a 7 seat car or 9 seat van/minibus.
Even if you were going to travel without the special car seats, the European 5 seat cars will not be roomy enough to fit in 4 adults and 3 kids, of any age.
While the laws differ a bit per country, the law for Germany about the same as that for the Netherlands, and while Switzerland may have different laws, they will also make it impossible to travel with 7 in a car meant for 5 at most.
When you make reservations for a car, make sure you tell about the age of the children, and that you need special seats for them.