It is a reasonable thing to worry about, as Yunzi have had lead content in the past. You should use the link on the Yunzi page where ChiyoDad discusses this subject

As you are probably buying from a different supplier than ChiyoDad, it is possible you could be supplied with yunzi containing lead. I think the reasonable reaction to your wish to protect your child would be to buy the Lead detecting swabs that ChiyoDad used. Then use it on a couple of stones when you first open the box of stones.

Depending on your social / negotiating / communication skills, you may be able to use the lead detecting swab on a stone in the shop.

You have to make the risk judgement yourself, not rely on the advice of a random stranger on the internet who is not qualified in the subject of testing for lead in stones. A simpler approach may be to avoid the risk and just buy Korean or Japanese double convex glass stones e.g. these: het paard glass stones