Just an idea.. but you can get some pretty good deals at butlins if you book at the right time of year! They had a half price thing on in earlier in the year and I booked a weeks holiday in a silver bungalow (apartments are 3 stories so i paid a bit extra for aground floor bungalow) for 2 adults and 3 children and it cost £250 instead of £520! You pay £125 deposit, then have to pay the rest off whenever you want between when you book it and when you go!
Obviuosly with just the one little one, it would be a LOT cheaper, my sister went in may and paid just over £100 for her, OH and a toddler!