I had thought at first that maybe they were drawn to Old Man Willow. The fact that Sam wasn't drawn that way made me think that Willow was doing only the sleeping part. After reading your comment, however, I went back and re-read the passage. The only hint I got about drawing the hobbits to him was this:
Frodo lay for a while fighting with the sleep that was overpowering him; then with an effort he struggled to his feet again. He felt a compelling desire for cool water.Did he [Bombadil] just "happen" along at that time or did he hear their call for help? Or did he perhaps learn of their presence from the trees themselves?
For what it's worth, I think it's just one of those master of his territory things. Tom just knows what's going on in his yard!
Very true... Tom, it seems, can know of things that are happening far away, and can come quickly to help when called! And help was needed... either Old Man Willow, or the river, or the whole forest were conspiring to lull them to sleep.
Sam sat down and scratched his head, and yawned like a cavern. He was worried. The afternoon was getting late, and he thought this sudden sleepiness uncanny. ‘There’s more behind this than sun and warm air,’ he muttered to himself. ‘I don’t like this great big tree. I don’t trust it. Hark at it singing about sleep now! This won’t do at all!’