front passenger airbags..
both are approximately 60 lbs...
Thats small enough you need to be careful about the airbags with the dogs riding in the front seat. I'm not sure if I'd strap them in where the airbag could hammer them. Alot of people dont' think about the airbags in their cars, but we all need to. If an airbag can seriously injure a 100 lb woman, it can, in theory, practically throw a 50 lb dog through the back window. Its something to think about.
Unfortunately your alternatives for the back seat arent' that good if its a bad wreck. The dog seat belts I'm familliar with only work with shoulder strap type seatbelts, which might not be in all cars back seats.