I decided a while ago that holidays with kids are, if finacially possible, about making things easy for yourself. Otherwise by the time you've got to your destination you may have saved £100 but you're all ready to kill each other.
Anyhow my thoughts!
I've used holiday autos for the last few years and they've been great. Even had a free upgrade this year! Don't order the smallest car if you can help it. Don't think you'll be able to pick up your car half an hour earlier than agreed (an issue if your flight times take you over another day car hire) you'll just be sat there for half an hour with two small, hot, bored children.
I do take our own car seats. It's an extra £40 total I think with Ryanair. Holiday autos only provide you with little boosters and I don't see why being on holiday suddenly should change my view on car safety! It's really no hassle taking your own.
Yes yes to seat reservations. Not being to together would be a nightmare - i've seen them do it to people.
Ryanair are great in my opinion. They are cheap and reliable and if you pay extra for the priority boarding can get seats no problem. I'm unwilling to pay an extra £500 to be smiled at
. However, cost your flights including all the charges so you know what saving your making before you book.
Take as many bags as your comfortable with. We took three hold bags this year for 9 days camping. Again, I am not one of the 'fit it all in a carry on' brigade. I recommend booking two bags. You can always add a third via their website so long as you haven't checked in on line already.
About the booking:
If dates are flexible on things then have a good look around first for flights. Sign up to the holiday autos emails - they often have deals like free extra driver etc. These deals don't last long so book when you can.
Car sick kids? - beg/borrow/purchase in car DVDs and take them with you. You might be happy looking at the view but at 4 and 7 your kids will be complaining and you'll be getting stressed if it's a long journey the other end.