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I have also diyed several Duplo build buckets off brick link...that is a fun gift to make!
If the little one is into princesses perhaps the Disney Juniors Cinderella #10729, Juniors Little Mermaid #10723, or Juniors Frozen #10736 - or if you're going the Duplo route #10855 (another Cinderella).
I too would echo any of the Cars 3 Juniors sets for the 5 year old, maybe #10742. It just seems like the perfect fit.
I think that maybe the #10723 and #10742 combo would work as they're both on the smaller-ish end?
To get them both something maybe one of those Juniors suitcases? Then you could carry it separately too LOL. Can't really see one that might suit both their needs, though you could always get one and then supplement with other minifigs (from what I remember there's a lot of room in those suitcases for other bricks n things!).
All that is to say that the Mom might appreciate a larger Duplo set for both to play with-it and also don't underestimate the play value of Duplo to older children. My daughter has quite a large collection of Lego as well. Duplo bridges the age Gap in between my two children quite well!
My experience: I have one boy who just turned 4, and he's been playing with Lego pretty consistently since he turned 3. We purchased #60051 (High Speed Passenger Train) for him for his 3rd bday (he really wanted it), and it's been Lego pretty much ever since. He still plays with Duplo on rare occasion, but if the Duplo's disappeared, he probably wouldn't care much.
Based on my experience, I would just buy a couple of Lego sets for the 3 and 5 year old. 3 years old is old enough for Lego, and 5 is too old for Duplo. Plus, Lego sets are generally smaller, so they will fit in your suitcase better (if you haven't already left for your trip).