I can't believe all of the fanciful and, frankly, utterly ridiculous scenarios that people are dreaming up... when it's perfectly obvious and straightforward to anybody who is familiar with the classic conundrum about the man who has to get a fox, a chicken and a bag of grain safely across the river in his rowing boat.
She's going on holiday to a small island nation where her DGD lives on a combined coach/ferry journey. Her DGD is crazy-mad about Sugar Puffs, but sadly you simply can't buy them where she lives. The suitcases are therefore stuffed to the gills with Sugar Puffs that the neighbour has bought from her local supermarket, clearing their shelves completely - emptied out and loose of their boxes, as you can cram far more of them in that way.
The problem is that she has a pet dog who is also a big fan of Sugar Puffs. He is also going on the holiday with her, to visit the family who live there, as they love him and he loves them. Naturally, as a dog, his sense of smell is top-drawer - so he can instantly smell the Sugar Puffs through the fabric of the cases, and is a strong and determined mutt, so he would easily be able to tear his way into them overnight, if they were in their own living room, and snaffle the lot well before dawn.
Therefore, the dog and the Sugar Puffs can never be left alone together - hence the cases need to stay overnight in next door's living room, which the dog cannot access. This part is key.
It's only by making sure that the cases go safely into the separate luggage hold of the coach, whilst she watches her dog like a hawk and takes him with her into the passenger seating area, that the Sugar Puffs will survive the journey intact.
Of course, when they arrive there, her DGD is going to be so excited to see both the beloved dog and the Sugar Puffs that she will want to scoff every last Sugar Puff within the hour, and maybe even share them with the dog in one massive Sugar-Puff-eating frenzy - which is clearly not a great idea for the health of either of them... but that's a whole new conundrum to be resolved separately, and one which OP can sadly have no logistical part in facilitating.