I've just got two little chaps, who luckily were neutered, vaccinated, flea and worm treated last month, so don't know too much about current costs.
The vet I intend to use charges £30 for an appointment and £70 to include a health check for vaccinations. You can't not buy a few toys, but don't spend loads, the two things my two like most are ping pong balls (get 10-12 as they get under everything) and old shoe laces. You can buy things like bedding, but they'll be more than happy with a chair or your bed (if they're allowed)
I've just got them on Felix pouches for now, they're eating 7 a day (12 cost £3) and a little dry. They're five months, so they're food consumption will go down as they get older.
We didn't have cat insurance with our previous two, but just put something aside each month, we still have £2500 in the pot which will be there for the newbies, which will be added to each month. Obviously you'd have to make the call whether to have insurance or pay as and when.
If you can afford it and would enjoy the company and fun, tell your husband they are so worth it!! Also, it helps teach the kids that animals need to be treated well and look after, but also will hopefully build up their confidence with them.