The duffel trick is great, and I always use it, but it will only work when you are not up to the limit on your max. number of checked bags. If the baby is being lap-carried on the plane, that max. is 6, and you're already right up against it just on what you want to check, and you didn't even count carseats and a stroller. So ... stop right now, take everything out of the suitcases, and start over!
Personally, I would go with just the 28" + the diaper bag, and one carryon containing valuables and the 3 yo's toys for the plane. If you want to go generous and add the 24" as a checked bag you should should be perfectly fine, but ditch the rest of it.
DO NOT PACK GRADUALLY INTO SUITCASES! They hide content once it is in, causing duplication. Pack on a big flat surface like a bed or table, do your checklist, and then have ONE person put everything into suitcases all at once. As you are deciding what to bring, pack the children's outfits into ziploc bags and squish out the air. You and your DH should pack for no more than 5 days, if you are there longer you should do a load of wash. Make sure that every item of clothing matches at least 2 others, so that you are not packing "outfits" for the adults. No "matching shoes" either -- bring walking shoes and water shoes, but 2 prs. ttl. per person should be enough (no shoes at all for the baby; socks will do.) Unless you already have ressies at V&A, no dressy clothes; one pr. of long khakis for DH and a light skirt for you will be nice enough for any other place you might go (you get 1 extra pr. of flat sandals if you bring a skirt.) All of the jewelry you are bringing should be on your body, and it should be real, so that you won't set off the metal detector at security.
Make up a separate family travel bag of toiletries, and bring small sizes; the only extra for the kiddies should be no-tears shampoo, which you can use to wash the whole baby. (Beauty supply shops sell empty 4 oz. bottles cheap, btw.) Unless you have really sensitive skin, you and your DH can probably use the hotel toiletries, though a small bottle of chlorine-removal shampoo is a good idea. Unless you take Rx narcotics, you can put any Rx meds into pill cases and leave the original bottles at home. Any other meds should all be able to fit into one sandwich baggie.
The diaper bag must contain a change of shirt that will fit either you or DH (in case one of the kids throws up on you), a spare change for both children, large ziplocs, a good-sized bag of wipes, and enough food and diapers for the baby for a 24 hour period, just in case you get stuck in an airport. Also, snacks for the 3 yr. old., for the same reason -- when airports get shut down, the food disappears fast. The carryon should contain several entertainment items for the 3 yo, medications, + any cameras or computers you have, a change of underwear for both you and DH, and your paperwork. Use a small purse for yourself, and put it inside the diaper bag. Carry your ID (and nothing else except maybe a spare binky) in your pocket while travelling.
Either go to the store once you arrive for the extra diapers and water, or use a delivery service like wegoshop.
Thanks! I agree with all the pp that you can ditch the H2O, formula, diapers and wipes and get it when you get there! Another suggestion I read somewhere on these boards is to ship diapers, wipes, etc to the hotel. 
Each bag gets the shirt, shorts, underwear, socks, etc. It is so easy to throw a bunch of ziplocks in the suitcase! Then I throw in the light coats, a couple of long sleeve warmer shirts and long pants. I found it saves room for other stuff doing this too and each morning you can grab a bag for your baby and toss your older one of his and their complete outfits are ready to be worn!
