I think that talking to the kids might be a fine idea. Inviting them to ride with you, even with asking the parents first- might be seen by the parents as an odd thing to do. I myself would have second thoughts about letting my kids go riding with a strange adult, even a woman. But their being Japanese immigrants makes it even more tricky, as Japanese people tend to maintain more formal boundaries (so my Japanese sister in law tells me). I can also easily imagine that their first reaction might be worrying that their kids had done something wrong and were creating trouble. The kids speak better English, so your communications with them will be much less likely to be misunderstood. Maybe the simpler you keep it the better the outcome under these circumstances.