Christmas is, undoubtedly, any child's favorite
If you are a parent yourself, you will understand the absolute gratitude that the parent of any child would display, when you gift the children a play tent. Especially in a house with siblings, energetic, boisterous play can be quite the norm. A play tent gives the children a wonderful way to restrict their energy to one section of the house (hear the 'hallelujah' from the parents here!), as well as give them a new space to feature as their secret hide-out, magical kingdom, or any other place their imaginations may fancy.
A ukulele, a small set of toy drums, a miniature keyboard or xylophone, a mouth organ, even just a tambourine - the list is endless! Kids are, by nature, noisy little creatures, so it wouldn't hurt if you were helping them channel all that noise and energy into a more pleasant outlet - though it is advisable that you check with the parents, if the kids aren't yours, before you go ahead and buy a mini drum kit!
Give a kid a heap of brightly coloured blocks that can be manipulated to create a multitude of things, and watch their imagination come to life. The best part about something like a Lego set is that you can vary the kind depending on the age of the child - more basic blocks for a younger child, with more complex sets with tools and things, as you go up in age.
If you know the children you are shopping for fairly well, endeavour to procure for them a costume of their favourite action hero or television/movie character, and you will be their favourite aunt or uncle for life... well, at least until the fancy passes! Every child has that one (or maybe even multiple!) fictional character that they admire and aspire to be in their fantasies, and a Zorro cape or Wonder Woman costume for Christmas is guaranteed to be a sure-fire hit.