I certainly would have said something to the child. Mainly for his own safety. If someone took off with my DS's bike he would go postal, and a scene from Terminator 2 would quickly follow.
However, it's terribly rude for the mother to just let that unfold. You don't just grab someone elses anything and take off with it. You ask.
By saying something kindly but firmly you are also showing your ds how to handle those sort of situations gracefully.

Playcentres suck for those reasons. I only take my kids in the mornings when it's quiet, but if that situation bothered me I would speak to management rather than take on a clearly oblivious parent. I usually say something to the older kids running amok in the little kids area. With a smile though.