>> am keen to understand what are the key differences between the First Class and Second Class travel in the Rail. We are travelling with 2 senior citizens, 1 toddler and two adults
The typical configuration of seating in 1st class carriages is 2 seats on one side of the aisle, and 1 seat on the other. In 2nd class, there are generally 2 seats on each side of the aisle, but you never feel squashed in. There is a bit more leg room in 1st class, but legroom is perfectly adequate in 2nd class.
First class tends to be less crowded, and is often used by business travellers, who use the travel time to work. Such an environment might not be ideal for an active toddler ;-)
>> so ideally need the seats to be together, comfortable
All seats are comfortable in both classes. Whether you can sit together depends on how crowded the train is. May is the low season for tourists, so I can't imagine trains will be crowded.
>> food and toilet available, enough luggage storage
If food services are available on any given train, they are accessible by both first and second class passengers.
There is information here about onboard catering:
Many stations also have food outlets where you can buy food and drink to take with you on the train. You can find out about the services offered at any particular station (including food outlets, luggage lockers etc) here:
Toilets are in each carriage, both first and second class.
There is sufficient room for your luggage in the train, either on racks near the doors, in the space between the seats, or overhead, depending on the individual train.
>> Is First Class available in all trains?
Most, but not all. Travelling 2nd class on trains in Switzerland is a pleasant and comfortable way to travel.
Some trains also have a special family carriage in 2nd class, which is always the first or last carriage on the train, and some of these also have a small play area on board.
This information about travelling with children might also interest you: