Ask Mr. Smarty Plants

Although Sophora secundiflora (Texas mountain laurel) is described by Poisonous Plants of North America as "Causing Only Low Toxicity if Eaten", The North American Guide to Common Poisonous Plants and Mushrooms by Nancy J. Turner and Patrick von Aderkas (Timber Press, 2009) says that the seeds are "halucinogenic and highly toxic; potentially fatal in small quantities to children" and "One seed, thoroughly chewed, is said to be enough to kill a child."  You mention that the child has no way to open the pods; however, as the pods dry they will split open on their own to release the red seeds and those seeds would be very attractive to a child.   Even if they weren't poisonous, there would be a choking risk for a small child putting the seeds in the mouth.  And, even if the pods didn't open on their own, I think it would be extremely risky for a child to put the seed pods in its mouth.  You would be surprised how strong the teeth and jaws of small children can be—strong enough to crack the seed pod, perhaps.

Here is a link to more information from Sweet Medicine Shoppe.com.

I would highly recommend that you do NOT let a small child use the mescalbean seed pods as rattles.  It would be an extremely dangerous risk for the child.