Another ETA: As a clinical psychologist, I worked with sex offenders for two decades, so I'm familiar with the way they operate. Yes, offering gifts can be part of their grooming strategy, BUT the man in this situation is not singling out this particular child alone for the cards, and he is not doing this in any hidden, secretive way outside of the watchful eye of teachers and other volunteers. BIG difference and a very important one to teach kids. Most schools these days use small, inexpensive tokens, tickets, cards, and such as a reward when kids are observed being kind or displaying some other good behavior. We should be teaching our children the difference between accepting a card or token that is given in a school setting and sanctioned by the teachers/school and those gifts that are offered in unknown, unfamiliar situations.
Per this poster's own "first rule of thumb," her son should have run in this situation. That would seem a little drastic on the school playground. At some point, we have to help kids see that if the school has allowed these other adults on school property to help teachers, and if the teachers know who they are, they should be considered safe persons.
Eventually, every child's sphere of comfort will have to expand to include relative strangers who will be part of the child's social life in a variety of settings. There will be coaches, religious school teachers, scout leaders, and other volunteers in these settings etc. Many of these people will be unknown to the children. There's no way for us as parents to take that factor out, so it's incumbent upon us to teach kids about being keeping themselves safe AND learning to incorporate new people into their lives.
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Haven't read any of the other responses yet, but I'm thinking your reaction is out of proportion to the situation. Of course it's good to be cautious and teach children how to avoid danger, but you can also go overboard by teaching them to be frightened and suspicious of everyone and everything. It's understandable coming from the lens of your experience as a child, but I think your radar may need some recalibration.
This man has been approved by the school administrators to be on the premises as well as hand out these baseball cards. I don't see the big deal with this. It's not like he's some random guy in the park who just wandered up to your child. That's where you have to differentiate between your traumatic experience as a child and normal, everyday interactions with regular people. Then you have to explain these differences to your children and teach them about risk factors in various situations.
This case is different because it doesn't take place in isolation. Rather, it takes place in public, with lots and lots of "eyes" on the situation. He isn't trying to lure your child away with gifts. You've got to teach your children these distinctions or they are going to become suspicious of everyone in every situation, and that is not a useful life skill or heathy way to interact with their world.
While it's up to you whether you decide to let your son accept the baseball cards, you children will be better served if you teach them how to evaluate low-risk vs. high-risk situations so they can develop good judgement about people and be better prepared to handle social situations independently as they grow.
Hope this helps.
ETA: It might help you to know what your school's policy is regarding volunteers. In our school, NO ONE outside of staff gets in without logging in on a computer which takes our picture and then prints out a name and photo badge that must be visibly worn at all times. And then, you still have to be "buzzed in by staff who make visual identification of you. If you know what your school's policies and procedures are, it might help you feel a little more relaxed about who's coming into your children's school, and that information will help you to present a more relaxed, reasonable explanation to your children regarding school volunteers.
J. F.