All the big companies do free child places with two full fare paying adults, so look at First Choice, Thomson, Thomas Cook, Jet2 Holidays, Holiday Hypermarket etc. However, most of the free child places get snapped up when the holidays first go on sale which was April/May this year for summer 2014, so many of them will have gone now but check anyway as you never know. Jet2 Holidays still have some free child places for single parents travelling alone with kids, so check on there as they may have some left.
FC and Thomson are great as you can save holidays that you like to a short list so you can watch as prices rise and fall, then strike when the holiday you fancy drops in price. Have a look at the Balearics as well as mainland Spain as it's still only a couple of hours flying time to get there. As I said above, prices seem to drop just after Christmas as people are too skint to book, and about 4-6 weeks before travel, when they're desperate to sell their remaining holidays.
Don't forget when looking to try putting in different departure airports (if there's more than one that's easy enough for you to get to), as it can change the price quite a bit.
Definitely book online rather than in branch, as that's where the cheapest prices are. I once had a quote in a branch and when I got home to check it online, I could get all inclusive for the price they'd quoted me for half board! Also don't forget to go via Quidco or Top Cashback when booking, as you'll then get a bit of money back a month or two down the line.
Don't forget to include the cost of airport parking, choosing seats together (if you want to - I always do), increasing your luggage weight if you need that, travel insurance etc, and don't forget to apply for your EHIC cards online before you go (you'll all need one, including your DD, but they're free - some unscrupulous companies charge a fee but from the official site they're free so DO NOT pay for one). Don't buy your travel insurance from the holiday company as it's a rip off, so look elsewhere for that once you've booked. Also bear in mind that unless you book all inclusive you'll need to pay for your food and drink while there.
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