Ask me about the 2 year old in 3 weeks!
Seriously, the 2 year old is cheaper than the 6 year old, I assume because the 2 year old cannot go to the kids club. They have to be 3 and potty trained. The 2 year old can go to the nursery for babysitting, but there is an additional charge & the hours are limited.
I think if it were me I would wait until both kids were over 3 rather than having to juggle one kid in the clubs & one with you or in the nursery.