I got married with DS aged 3 and DD 11 months. My DD was also a bad sleeper, but she wouldn't settle elsewhere so just had to put up with lack of sleep and used lots of concealer expertly applied by my sister to disguise the shadows under my eyes!
We had lots of family staying in holiday chalets nearby, as at 9am DH carted off DS there, and they both got ready with all the male side of the family over there, while my bridesmaids came over and helped me with DD while I got dressed etc,- we managed to get DD to nap at 11.30 after a good snack so that I had an hour to finish getting ready in peace. Then we woke DD, quickly gave her some nosh, got her chnaged, and then headed off for the ceremony which was at 1.
My DCs were kept very much involved- I carried DD and my flowers down the aisle, and DH & DS were waiting for us. The DCs stayed with us during the short (civil) ceremony as wouldn't have settled really with anyone else, especially is such a strange situation! I love the fact they were there with us while we took our vows- both were very quiet and good- I think just stunned by what was going on. After the formal bit, we'd arranged some snacks for everyone at the same venue, it was pretty informal and lots of kids were there, so they ran about munching shortbread and crisps for an hour or so while we had photos taken etc.
Then we drove to the reception, and DD fell asleep in car- we had friends take turns keeping an eye on her in the car for about 40 mins so she had some decent sleep before the fun of the afternoon/evening. Had the meal, then I took both DCs home with my best friend and her DD, got them settled into bed, changed myself and reappeared at the reception at 9pm! Had a babysitter look after the DCs once they were asleep, and then my MIL was staying at ours, while we stayed in a hotel, later that night. I was then able to let my hair down!
This was all made possible by lots of help and having our wedding very near home, but I'm sure you'll find a way of fitting DD into the days events. Oh and we set up a corner of the reception room as a kids area, with a few toys, DVD player, cushions etc., so that they could be entertained a bit early in the evening. Our was a very child-friendly wedding as we had to make it that way, and informal too, but I'm sure you'll find it will all work out fine!
Enjoy your big day!