breed...we have two boys 7 and 8. I don't want a large dog or a really tiny one. Don't want a dog who moults/dribbles a lot or one who needs hours of walking each day.
Think about what you can give a dog, and then pick a breed that fits your requirements. Don't try and fit an unsuitable dog in to your life.
puppy or rescue / boy or girl?
Rescues probably won't re-home to you with your working hours, to be perfectly honest, so you'll probably need to go for a puppy. I don't think gender is important but do remember bitches go into season and you need to plan accordingly.
what would we do when we go on a day trip i.e. theme park?
You'd need to plan this well in advance and book your dog into kennels or daycare. You can't leave a young dog home alone all day. Maybe when older you could leave the dog and arrange a walker, but be aware that many walkers and daycare providers don't work weekends, or if they do, they get booked up weeks in advance. Or, get family to look after the dog for the day.
What would we do when we go out at night together and kids go to GPs? Rare but happens 2/3 times a year.
If the dog has been home alone all day, you'll need to plan care for the dog too. Either the dog could go to grandparents too, or a home boarder or kennels. You can't leave a dog all day and all night - they're very restrictive in that respect.
How much is flexible day care while at work? Kennels for holidays? Walkers?
Daycare is around £20-30 per day. Walkers are about £10-15 for a group walk. Some are flexible, but licensed boarders have limits on numbers so they may not be able to accommodate lots of changing days. I'm a walker and while I'm as flexible as I can be, it's not always possible to change times/days if I have lots of dogs booked in.
How much will food cost and vets?
That depends entirely on what breed and diet you choose. Bigger breeds are more expensive. Some breeds will cost more in insurance as they're known to have health problems. Don't forget flea treatment, worm treatment, spay/neuter and injections too.
What time do they get up? Do they have to go out for walks straight away? What time do they go to bed? Sleep all night?
Your routine as a puppy will be different to an adult, and a lot of it will depend on your routine. Mine sleeps until I get up - that could be 6am or 10am. Then it's about an hour before he goes for walk. He walks anywhere between 1-3 hours a day. He sleeps on the sofa from about 7pm and comes to bed when we do (around 11pm) and sleeps all night. That's been his routine since he was about one.
should we borrow a friends dog for a day to see how we all get on with having a dog at home?
You could, but having a dog for a day is VERY different to having one full-time. It's a big commitment. It won't really give you much idea of the commitment and work involved, I'm afraid to say. Walks everyday regardless of how shit the weather is, feeding the dog, letting them out to the toilet, training, cleaning up accidents, stopping them eating your furniture, dealing with the bitey puppy stage....it's hard work and you have no idea just how hard until you're thrown in the deep end!