What we call the Pledge of Allegiance wasn't created until the 1890's and wasn't officially adopted by Congress until 1942 so far as the pledge we know today I'd guess they didn't have much of an opinion. Interestingly Francis Bellamy who wrote the pledge was an open socialist. So far as the founding fathers in specific, if you look at early America there was a strong distrust of central government and a emphasis on states rights. This is why early on the U.S. had almost no standing army and relied more on state militia. If you really want to get an idea of the feelings about central government (and the founding father's reaction to the idea of a pledge to that central government) I would read up on the Federalist vs. anti-federalist debate which took place between people like Alexander Hamilton and John Adams vs. anti-federalist like Sam Adams and Thomas Jefferson.

TL;DR - Pledge wasn't really a thing until the 20th Century and the founding fathers would likely have been against a mandatory pledge to a central government.