On our last holiday the in-laws came too which was handy,as we booked and paid for ourselves and got one free child space for dd,and the inlaws booked for themselves and my ds,and they also got a free child space for our ds.
If it was just us going without pil's,we would have had to pay for one of our children,as you can only get one free child space with your booking.
Go either all inclusive or self catering,as eating out all the time will add up.
If self catering,don't buy your food from tourist shops by the hotels as they are normally a bit more expensive.
Term time holidays are cheaper than half terms/school holidays.
My friend looks for a good but cheap holiday in a travel agents,then goes into a different travel agents and asks it they have a similar holiday they can do cheaper.which they do,so he goes back to the first travel agents and knocks them down on price,and again at the second travel agents.he's been known to do this a lot and always gets a good bargain.
If that's not your style,book your holiday online yourself as travel agents put a small fee on top of the holiday.
Bring nappies,wet wipes/suncream with you as they're normally expensive abroad in shops near the hotels.