Boden? You should be ok with that age gap but I remember looking for matching for dd1 - 3 and dd2 4 months and it was really hard because so many baby collections only go up to 2. I dodn't know about boden then!
They will be 5 months and 22 months and the only children at the wedding. I'd love them to wear the same dress but possibly in different colours. I just can't find anything nice that's for both babies and toddlers.