To be honest, I think that you're going to have a hard time backing out without making yourself the bad guy.
If you had offered to pay for her kids, and SIL had tried to invite herself, that would be different. But for a pay your own way trip to a cool location, wanting to accompany your own kids rather than send them with a relative is a pretty reasonable request.
But as it is, you suggested both the trip and the destination, and it's a pay your own way trip, backing out now for nebulous reasons is not going to look good. If you've visited the same location recently with your own kids, then saying that your finances are tight and you can't manage it isn't going to be a very believable excuse.