I agree with highlander about the stress that travelling puts on older people. I know some are real gadabouts, but others IME get really reluctant to take the driving wheel or brave a busy train station.
My mum, though physically and mentally fit, got slightly agrophobic as nas she got older and busy, strange situations brought on panic attacks. I know another elderly man, who used to drive happily all over Europe but has sold his car because his eyesight was failing just a bit, but enough for him to realise he is not the same careful driver that he was. Also, IME elderly people may have bladder problems that they don't want to discuss but stop them wanting to go away from home territory as much.
I was thinking about this more yesterday. I feel that driving to your parents 4 times a year is not too little. However, as you're asking for opinions, I think somehow when you get there, you should stay for more than a day. Not just for your own sake, but for your parents sake too. If they can spend two or more days at a time with their grandchildren, they have more chance of getting to know them. Everyone will be more relaxed, too I imagine. After a 4 hour drive, you and your children are probably not at your brilliant best. I don't know what your children are like, but my son, especially when tiny, got hyper after a journey. Being in a new house and seeing his grandma added to his excitement, so he was prone to be a bit naughty and attention demanding on the first day. After a nights sleep, he setttled down more and I felt tons better, too. I used to love the day after we arrived - everything fell into place.
Ahh...nostalga trip for me. My parents are both dead and I do so miss the visits to them, mind you, I only had a 2 hour drive. I look back with regret at some of the times I cancelled trips to see my mum, not realising at the time how ill she was (she didn't know either). I used to resent the fact she would hardly visit me in London (even when she was a fit, active 70 year old). Really stubborn she was. I gave up trying to pursuade her. I hope you have more luck pursuading your parents, but feel that if they don't change, you will have to bend to their ways. Unfair, but that's life.
I am assuming here that you like your parents and they really want to see you, but of course I don't know your full story. I am puzzled why you say you could never stay in their house. That's a difficult one, if that is really how you feel.