We've been trying to convince DSD to get them done for about 18 months - the plan was it could be a 10th birthday present..... we took her once... she freaked out.... (obviously we didn't end up stabbing her then...) and it was third time lucky.... it appears her mother had been telling her it would "hurt so bad and you'll freak out and it will be terrible... stupid woman...

anyway DSD is now SOOO happy to have them (she has been desperate for ages!) and she is looking after them herself and so far no problems.
I had mine done for my 9th birthday and I think I too was more than capable of making that decision and looking after them.

I don't see any problem with getting them done as a baby.... but I think the bigger issue you have here is that you and DP have very different opinions. That's something that you both need to come to terms with first - my feeling would be that if he doesn't want it, then you probably can't force the issue. Maybe you can convince him - or negotiate, but I agree with the others who have said not to do it behind his back - that's just fraught with danger!!