I agree you need to step carefully when in another grandmother's territory.
But that aside, early first projects kids enjoy, especially if they get to pick the fabric, are the pillowcase as mentioned. Also try a pair of sleeping pants/boxers that can be teamed with a store-bought t-shirt or tank top. The sleeping pants have long simple seams and they'll learn to hem and make a waistline with elastic casing. And if it's not perfect, who cares...they wear them at home to bed! If they are interested in clothes, I'd then do a simple skirt. Piecing a simple 9-patch pillow with colors to match their room is fun. And...if they don't have a very big window in their bedroom, you could do very simple curtains that gather onto a rod. They'll have a lot of pride in creating something for their own room. Then maybe on to a lap quilt!