Saint Bernards adore children, they love to be involved with all your family everyday activities.

If I were considering a Saint Bernard...

My major concerns would be:

Providing the proper balance of exercise. 
Young Saint Bernards need enough exercise to keep them lean and healthy, but not so much that their soft growing bones, joints, and ligaments become over-stressed and damaged. 
Adult Saint Bernards need more exercise to keep them in shape, but not in hot or humid weather for fear of overheating. 
The proper amount of exercise can be difficult to regulate in giant breeds.

Since you have to minimize their exercise, young Saint Bernards can be very rambunctious. 
They will romp with uncoordinated gawkiness all over your house. 
You need to substitute extra quantities of companionship and supervision. 
Otherwise, left alone, young Saint Bernards become bored and destructive -- and their powerful jaws can literally destroy your living room. :0)

I strongly recommend that you get your Saint Bernard involved in obedience classes at the intermediate or advanced level, or tracking, or pulling a cart or sled, or even just hiking and swimming. 
This is a working dog who needs something interesting to do.