Lisalisa, they have rather bulky pushalong cars that you acn hire for the day (about £5 I think) but it may be an idea to use one of these for your 2year old dd and it take all picnic stuff/towels/extra clothing etc. Just thinking that your dd may love her special car so much that she may not get out of it too much or at least want to go back in it after any rides.
There is a big 'waterworks' area where kids can play in fountains etc - hence the need for towels and spare clothing!.
Also, with the age gap and the time spent queing for each ride, you may want to split up for some of the time so that all the children get to go on as many rides that are appropriate for their age as is possible!
If the weather is nice, take a picnic as the queues for food are not much fun either!
Have a lovely time!