Return an iterator in ForkChoice.getChildren method

[dadepo]

    fn getChildren(
        self: *const ForkChoice,
        slot_hash_key: *const SlotAndHash,
    ) ?SortedMap(SlotAndHash, void) {
        const fork_info = self.fork_infos.get(slot_hash_key.*) orelse return null;
        return fork_info.children;
    }

This should be updated to return an iterator instead.