Congrats on getting to the budgeting stage before having kids! We waited until afterwards to get serious on finances which added a layer of complexity.
On many of these areas it will be hard to come up with numbers because they are specific to the area you live. Also, with kids, planning can be very difficult. Our kids have been cheap to clothe, feed, entertain (hand-me-downs, cloth diapers, lots of free activities) but childcare costs were crazy for a year or two. After my 2nd was born, our childcare situation fell through at the last minute. We ended up with a nanny as we were unable to find a convenient daycare that also matched our schedule and low-car lifestyle. Last year our childcare/preschool expenses were in the $2k range (for only 3 days/week) which was more than our mortgage. Yikes! This year my older son went off to kindergarten and we were able to find another family to share our awesome nanny. I don't regret the expenses because my boys love their nanny and I felt ok leaving the house each day but I would not have anticipated that kind of cost after having my second. My oldest son went to a home daycare for ~400/month.
Right now for our family of 4 I budget in Portland:
$50/month for kid supplies, field trips, clothes, occasional sports/swim lessons but we keep this simple
$1000 for childcare for 2 year old and after school care for 5 yr old from nanny
$600/month for groceries. Sometimes I can undercut this by $100 or so (mostly when the summer garden is producing) but after some experimentation I find this is my happy number for buying high quality meats, local produce, etc.
$50/month for gas- got a cargo bike last year and now this is generous
$40/month for car insurance (2001 Subaru Outback)
$100/month home maintenance, although I budget separately for major anticipated repairs
I don't know about healthcare. My husband has a government job with super cushy benefits so this is one area we don't currently worry about.
Honestly with friends, consignment stores, and a little creativity kids do not have to be so expensive. The key is not to fall into the idea that your kids have to do every possible enrichment activity. My boys mostly have a lot of open creative time and reading and the older one is thriving now at school.