Some changes we've had over the 3 years since our son was born:
Used to get £100 a week pocket money each, now get £50 each saving £100 a week
I used to spend £40 a week on fuel to get to work which went down to roughly £5 a week on maternity leave and is now £10 a week as I work from home saving £30 a week
Used to spend about £200 a month on take aways and eating out every month, now spend around £80 a month saving £120 a month
Used to spend about £80 a week on the shop buying easy stuff as we both worked full time and less time to cook, now spend about £70 a week saving £10 a week
Used to spend about £60 a month on gym memberships we never used, now spend nothing on them and just go for walks and to the park saving £60 a month
Used to not have a childs birthday and Christmas to pay for, clothes, toys, entertainment, playgroup, pull ups, wipes, toiletries, replacing many broken items in the house etc! Which I can't put a figure on but I would say our son 'costs us' around £300 a month to pay his playgroup, get him clothes, take him out, buy toys and presents etc.
So overall we have actually saved having a child! He will get more expensive though as he gets older. Right now we can get away with buying him a £20 pair of trainers and clothes from sainsburys/asda/matalan/next sale etc but as he gets older, he will need clothes with a bit quality to them, same with shoes and trainers. He will also have school meals or packed lunch, school trips and uniform etc but hopefully by then I will be working more, maybe double the hours I do now