As the others have said, more info would help.
Making assumptions, perhaps this will help.
If you are going for a week (the standard UK holiday break) and your girls are both still in nappies then I would suggest the following...
-enough nappies to get you to your destination (buy the rest there to save on luggage space)
-for each of the girls changes of clothes, mulitply the number of days you are away by 1.5 (ie away 6 days = 9 changes of clothes each) At that age they will be quite messy, so will likely have some days that require a change, but not every day hopefully!
-additional weather appropriate clothes each
if cold - rain jacket, hat, gloves, wellies
if warm - swmim suit, sun hat, beach towel, sunscreen.
Will you be driving or flying?
There is (hopefully) helpful reference material on my baby travel website. Including packing checklists and wot not.
Good luck with your journey. Have a fab time!
Hayley