Any tips for how to keep a duvet from bunching up or slipping down in its cover? There aren't any buttons or loops in the duvet itself and using something like safety pins looks tacky and might damage the duvet.
Usually a duvet shouldn’t slip in the cover. If it does, I found it’s often due to one of these three different causes:
The size of duvet and cover doesn’t match. The duvet should fit snugly in the cover without being cramped (-> leads to initial bunching) or extra space (-> encourages slipping around). For too small covers, there isn’t much to do except get a fitting one, with too large ones turning them inside out and sewing parallel to the existing edges can be worth a try.
The sleeper grabs only the cover, not the duvet inside, when covering themselves or turning around at night. Especially children with small hands physically struggle to get a good grip, we adults sometimes need to “practice” a bit until “grabbing a good handful” becomes automatic.
The materials of duvet and/or cover are particularly “slippery”. Especially synthetic materials are often very smooth. (Hint: check the texture ideally before buying, try to actually touch the materials.) If shaking out the duvet when making the bed doesn’t suffice, you may have to add some “fixing”: Sew a piece of ribbon (sturdy, washable, preferably cotton, also with a bit of “grip”) both to the corners of the duvet and the inside corners of the cover. Tie the corresponding ribbons together (no permanent knots, you want to separate them again next laundry day). This will of course also help in the cases above.
First: Dress the duvet with the cover opening at the foot of the bed. Of course you want to ensure the top edge of the duvet is at the top of the duvet cover when you do. This may leave a slight gap at the bottom which is relatively trivial. The tendency will be to grasp the top edge of the cover and contents
Second: Each time you make the bed is a good time to grasp the foot of the duvet and cover and - with a few shakes - fluff up the duvet and "re-set" the position of the duvet at the same time. The foot of the bed gives easier access to the full width of the duvet cover/duvet unlike the headboard where you must walk around the bed to easily adjust cover - sometimes repeatedly.