Absolutely take the opportunity! Sounds like a dream come true!! Plus, he's your Dad, so what more could you ask for?!! I don't know what exactly your responsibilities will be, but you will learn to work around your kid's schedules. Like during nap times or if they have a quiet-play time. If you will need to be on the phone or computer while they are awake, try to set them up with quiet toys nearby and take a break every now and then to interact and play with the kids. If you need to run errands, can you take the kids with you? Obviously, you won't be able to do that if its for a meeting or something of that sort, but if you are doing things like picking up dry cleaning or going to the store, you can bring em along. You might get up an hour before the kids do so you can do paperwork or scheduling (or whaterver it will entail), then take a break when the kids get up and have breakfast. YOu will find a groove that works for you! Will your 4 year old go to preschool in the fall? That will make it even easier!
If it were me, I'd take the job in a heart beat!! Good luck! I hope it all works out for you!