of course weather seems to be different from what it used to be - but according to my experience, April is still snow-time in the Berner Oberland. Perhaps it's already beginning to melt in warm years, so skiing wouldn't be a sure thing, but it would be rather unusual if it would be already full hiking season.

If you (your parents) want to walk, I'd suggest June as the earliest. Or you change the destination to some area on the south side of the alps like southern tyrole (trentino alto adige) or the lower parts of the Ticino, around the lakes e.g. But even there May would be better....

Baerbel in Germany