I agree with pre-warn. My DS1 had his 4yr vaccinations last week. I kept it simple, said he'd have two needles which would feel like a little pinch and would be over really quickly. When he asked "why" I said it would stop him from getting sick when he was older. I also said I'd take him out for a treat afterwards. I mentioned it 3-4 times in the 24-hours leading up to it, but without making too much of a deal of it as I didn't want him to get worked up about it.

He was pretty good when he had the shots, he sat in my lap and (as you do) I held him nice and close - the nurse was fantatstic too, which helped. She managed to distract him just before she put the needles in - the second one was hardest, as he knew what was coming, but as I said, the nurse was great which went a long way to make DS1 comfortable. She gave both him (and DS2!) a lollipop and sticker immediately afterwards

My boys have seen me have lots of blood tests during the past seven months, but I still don't think it makes much difference when it's them in the chair.

Good luck, but honestly the lead-up to the vaccinations (for mum) is worse than the actual event itself.