if you have the use of a computer you could take the pictures in colour and turn them black and white in an image editing program if you have one, might be a bit cheaper than black and white film and it's processing costs and waiting time. Don't know if it's changed in the last few years but most places have to send the films away to get processed and it takes about a week so take that into consideration if you have to have them done before the birthday.
Sounds like a lovely idea by the way :)