1. They should have spoken to you if they feel you need to use YOUR time differently!
2.You really shouldn't feel guilty..this is your life and you can live it as you wish. You should not be unwillingly/guiltily "squeezing out" your own precious "me time" to suit them!!
3. Why do they think that your time is their right to use/dictate to? They are seeing you as a solution to their stresses which is completely unfair. The only solution for them with you, is using anything that YOU are willing to offer!
4. Why do they see this is your "failure"? Why is childcare your job? Your husband is retired, he can do it IF he chooses to!
5. If they don't want to use childcare that is up to them ...but does not mean that they can use you for free instead!!
I think you know all of the above and I think you need to fight your own natural guilty instincts. You are a "giver", someone who helps, and have rightly been doing that for your dad and will be again. Don't let anyone take away your time unless YOU want to give it. It is very true that grandchildren are only young for a short time ...but I think as grandparents we all have to forge our own role according to what WE want and what we are allowed to do ...but what |WE want comes first.
Decide for your self what you want to do; tell your husband. Then I would talk to your son and DIL together; explain that you do not feel able to offer regular slots of time for care because you are working, you get tired, you are looking into future care for your father, you need some time to your self to relax. Tell them what you can offer, if anything. Then ...carry on living life as YOU want to!