Honestly, it depends on the bench, but most street benches follow a similar pattern. The gaps between timber or metal slats are usually around 1/2 inch to 1 inch wide—roughly 13–25 mm. That is tight enough for drainage and comfort, but not usually wide enough for a child’s whole hand to pass through.
However, the fingers are the real concern. A toddler’s hand may be chunky, but the fingers are slim. If a gap is between about 3/8 inch and 3/4 inch, a child can slide fingers in, then twist, fall, or panic. The finger may swell or wedge against a rough edge, and that is when it gets stuck. So yes, finger entrapment is possible, especially if the slats are warped, loose, or spaced unevenly. A full hand getting stuck is much less likely because that would usually require a gap well over an inch—beyond normal bench spacing.
What I would do is take a quick look with a tape measure or even a key. If the gap is around the width of a felt-tip pen or wider but still smaller than a child’s hand, treat it as a pinch or finger hazard. If the bench is in a playground, school, daycare, or park used by toddlers, ask the owner to check it against local safety standards. In general, safer designs either keep gaps small enough that fingers cannot enter, or large enough that a hand can pull out easily. Public benches do not always meet that ideal, so it is worth a quick check before letting small children climb or kneel on them.
Bottom line: a normal street bench gap is usually around 0.5–1 inch. It probably will not swallow a child’s whole hand, but fingers can get stuck. If the gap looks uneven or roughly finger-sized, treat it as a hazard and report it.