It is possible for them to break. From what I have heard, most breakage happens during storage, shipping, display and while washing the toy.
Borosilicate (Pyrex) is made to break into large pieces. I assume glass toys would break the same one, although I only have one. I take very good care of it. I put a thick wash cloth or towel down on the bottom of the sink before washing the toy and get the cloth wet, so that it will absorb any damage.
Also, glass should be inspected before any use. If there is any chipping or breakage, the toy should be discarded.
There were some problems with one brand of toy (I can't remember which one, but Carrie Ann started a thread about it a few months ago) which were breaking in stores while on display
! It was suspected that radical temperature changes were responsible.
Radical temperature changes, say taking one out of the freezer (which is NOT recommended) and holding it or allowing it to touch flesh can not only cause breakage but also induce frost bite.
Saying that one shouldn't put glass toys into the freezer, and Don Wands says do not even place the toy in ice water. Only COLD water. Or warm water. And obviously don't microwave or heat the toy in an oven.
Needless to say, glass needs to be treated with a lot of care and one must be a little more vigilant with care, use and storage than toys made of less breakable material.