If you did, it wouldn’t really be a big deal. Toddlers daily encounter playgrounds full of tiny rocks, wood chips, etc. which are more dangerous than the BBs. Swallowing BBs is comparable to swallowing cherry pits. It’s very unlikely to be life threatening. Typically there is no noticeable effect, or there might be some minor discomfort as it works through the digestive system.

I would be entirely comfortable letting my under 2 year old play with BB-sized steel or glass balls, under loose adult supervision, from a safety perspective (from a that’s-going-to-be-a-mess perspective I can well imagine avoiding giving kids BBs). More realistically, I can imagine letting an older child play with a box of BBs without worrying that a toddler would eat a few and die. Larger marbles are more of a choking hazard, and require closer adult supervision.

But swallowing magnets is much more dangerous.