I often have this dilemma as it's easier to shop for my DD than her twin brother and I want it to be comparable.
You have to decide what you mean by comparable. Is it the monetary value or what the child would consider the actual value?
If the kids don't know how much one thing costs versus another, you really need to find something that the younger ones will love as much as the older girls will love their trip to BBB.
If they are into legos, than go with that, but I've found that I don't have to spend the exact same amount of $$ on each necessarily. I do try for all their Christmas gifts to add up to the same amount, but if I find a good sale on something, that can skew it.
My kids will be 8 at the end of the year. I try to have one BIG (size) box for them to open at christmas along with other items. Last year, my DD's big gift was the Kit & Ruthie dolls from American Girl that I had won through Barnes & Noble. I bought DS the Rock Band set (on sale). So, size-wise, they were comparable, but not price-wise (as I didn't pay anything for the AG dolls).
Figure out what at WDW your little ones would really like, and that is your answer. Legos, My Little Pony fill a box, Mr. Potato Head fll a box, maybe a dress up outfit for your littlest DD....or even just a special stuffed animal.
Good luck!