Oh dear, 4 kids!!!
The transit sounds the best option. I have a Skoda, Joy, an estate, one of the nicest cars I have driven! but it is a company car and I don't like putting dogs in it as it is leased and has to go back soon.
I bought a Vauxhall Astravan, which is fantastic and economical on diesel, but I only have 1 deerhound and 1 terrier, my 2 eldest kids don't come in a car with me anymore. You can now get the 1.3 diesel version and at £35 a year road tax it is worth looking into, I have a 1.7 tdi and get 550-600 miles to the tank