That's a tough question Saburo. Probably the best way to handle it is, to look for awhile at the drawing and say, hummmm. Then ask them about it, and they will tell you what's going on in the picture.
Even children can detect insincerity, if you just look at it quickly and say, "That's a nice drawing." Or that's a beautiful drawing/picture, what is it? Or worse, "That's a good picture of a cow." ...When it's supposed to be a horse.
Saying you like it is judgemental. Children see right through this as bogus.
Best thing to do is take some time to consider the artwork, and let them tell you about it.
Les