Do you mean travel as in commuting and other essential travel - or holidays and leisure trips? If the latter, I guess you'd just instinctively drop or massively scale back on these if you had young children.
Apart from anything else, the excitement of a couple of adults booking a last-minute getaway and having a fantastic whistlestop visit very much loses its allure when you have to factor in all the tasks, responsibilities, limitations, stuff and added costs of having kids with you!
A lot of the budgeting in the 'can we afford kids?' scenarios seem to be people weighing up non-essentials that they currently enjoy with the costs of having potential children - which is very wise and good. It's just that, often, you can have either the luxuries OR children, and you just have to weigh it up and decide which is more important to you. Either choice is equally valid, but you very often simply cannot have both.
It's not just money, but time as well. You might be passionate about free or very cheap hobbies and spend all weekend and every free weekend doing them. Bring children into the mix, and they're largely gone (or at least heavily reduced) in the blink of an eye!