Well if you can't afford it they can't go can they? Maybe your parents can't afford the petrol or have some other reason that they don't want to drive down. Frankly since they are the ones doing you the favour I don't think it is too onerous an expectation that you should be the one to get your kids there. it s bit much for you to feel they should shoulder the expense of transporting the kids as well. Its not their fault you don't drive.
Like I said if you can't afford it then you can't do it, plenty of things I can't afford for my kids to do.
I don't know what you want people to say. If you can't afford it then they can't go - end of story. I don't think you'll find many people willing to say that your parents are being unreasonable though when they are offering free childcare and only asking you to deliver your children.
Well if its going to cause a row don't say the you can't afford to send them. Thats unfair on your parents as it will put them under pressure to pay. Just say that the timing doesn't suit and try and organise another time when you have had a bit of time to save.
I find the attitude that because your parents can afford it they should pay quite strange. I don't think things operate on the basis of espenses being paid by those who can afford them rather than those who incurr them. I don't go out for dinner with my friends and expect them to pay for me because they earn more.