I would be amazed if anyone has not had vast expensive costs related to children, starting with all the nursery equipment and moving onwards from thereon in.
We moved house months after DC1 was born and, although we have stayed in the same house, we have forked out on two extensions - one to give us more daily living space and the other to provide an extra bedroom when we went from 3 to 4 DC.
Car-wise, we changed the car we had when DC1 was born as it was not big enough for the buggy/pushchair and our dog. We then needed to change cars when changing from 2 - 3 DC. Most cars are not wide enough to accommodate three car seats in the back - DC1 was just 4 when DC3 was born. Having moved to a people carrier then, it at least meant we didn't need to change cars when DC4 was born, 7 years later!
Holidays are so expensive - I have a sister in Canada whom I used to visit every other year. All four children have been twice (DC4 was 9 weeks old when my dad was taken ill whilst visiting my sister - I couldn't leave her behind as I was breastfeeding and so she came with me, whilst Dad looked after the others at home). However, these are the only times DC have flown - the only package holiday we have had since having children was a week in Tunisia when I was pregnant with DC2. Holidays nowadays are either camping or a mobile home and often on sites where there are activities for the children. I don't mind this at all - the whole family have some lovely holiday memories and there will be plenty of time to travel abroad again once the children have flown the nest.
I think the costs that have surprised me the most are related to schooling. All four DC attended the same local primary school and the three eldest all attended a church school close by. Primary school uniform is fairly affordable, especially with most of the supermarkets offering cheap, hardwearing uniform. However, it is the constant demand for money for various events that really adds up - one week a mufti-day, the next week donations for the school fair etc etc.
However, primary school is nothing compared to secondary school. For starters, to equip DC with new secondary school uniform, including PE kit cost about £300 minimum. This was without school shoes etc. The greatest shock though came when DC1 started Year 9. The secondary school had Activities Week during the last week of the summer term - this usually involved paying for a couple of day trips during the week, along with different activities at school. However, in early October when DC1 was in Year 9, she brought home a letter describing the options for Activities Week that year. Essentially there were 5 options (although a 6th option was for the child to stay in school) and all involved going away from the Monday-Friday. A couple of options were UK based, one was a PGL type break in France, another a history based trip to Belgium and France and the fifth trip was a trip to Spain. Four of the trips were priced between £280 - £350 but the trip to Spain was in excess of £500. Not only were these trips the cost of a caravan holiday for our whole family, but it was done on a first-come, first-served basis. That was fine, except you had to send a minimum of a £50 deposit with the reply and then there were 2 other instalments, with everything due by early February. As a family who has a very tight budget, trying to find the £50 so that our child could go on a trip of her choice was very difficult - never mind finding the balance by the required date. At least with DC2 and DC3 we were prepared for the letter!
Having children involves huge expense and changes family dynamics. With 4DC we have little money and are not in the best place financially. If I could live my life again, there are some differences I would make, but I would still have my 4DC - no amount of money could compare to the joy they bring me.