We plan/hope to go in the spring/summer before Earl turns 8 so he'll be 7.5 ish. That would make Eddy 5 and the new baby just shy of 4. Hoping this will be a good compromise for them. We then hope to take them all to Florida when the youngest is 8, Eddy is 9 and Earl is 11, nearly 12 (before he starts senior school).
Same as you, we've got the age gap to consider but I'm hoping that these 2 trips will allow them to get the most out of it all, and we may even be able to fit in another trip before Earl leaves home (when he's 15/16) and the youngest will be 12ish.