Mars, up here they get MMR around 18 months and a booster pre-school. In my haze of the DTs 18 months, I believe they have already have had it (I had to check the red book!!!!) and I believe my mum came with me. I figured if they were going to be off colour afterwards then I'd rather they did it at the same time. They were fine.
DD1 had hers very, very late as it was around the time of the huge Andrew Wakefield paper.......she is well overdue the booster but I'm now concerned about her reaction to anything foreign following her Stevens-Johnston Syndrome experience.......zzzzzz......
In short, go for it, go for it all together in one as it were. Bribery wins through everytime. Do chocolate. Do Chuppa Chupp lollies. Do anything. Just make it easy on everyone.
TKB, you have your girls, you have company, you have hair and make up sorted, you have the dress, the man, the family, the day, the ceremony, the reception. I would join Mars with your little ones (in a very separate room with toys, books, bubbles etc) if they would have us (yes, including the baby of the make up woman, if she attends to my undereye circles in a private moment). I mean it. I don't do meet ups but I do do assisting in times of emergency and importance and this is now. When is it? Would it be helpful?