This is very cute!
I think it would be best to have the father and son sizes with individual quantity selectors on each product page.
So...
Dad $15 per shirt:
Size: Medium
Color: Gray
Quantity: 1
Son(s) $15 per shirt:
Size: Itty bitty baby
Color: Blue
Quantity: 3
[Add to cart]
This would allow replacement shirts to be ordered easily for dads as well as helping with the twin or more issue.
Site issue: The "Free Shipping On All Orders!" banner disappears when there are items in the cart. (I don't want to buy anything, but I was looking at how quantities work at the moment.) This banner should be present all the time. Also, your headers are a little awkward (two lines of headers, with banner above, and logo on second header), and your logo (nice design!) should be larger.
Also, on a lot of other sites I see links at the footer for contact form, return policies, info about shipping, etc.
We ship during business hours Monday through Friday. We use UPS tracked mail, which typically will take 5 to 7 business days for the continental US. [Map of shipping times from somewhere like this: https://www.ups.com/maps ] We do not ship to Uzbeckistan. Orders shipped to Canada will incur a $9.17 border crossing fee. Free gold doubloon included with each order of over 475 lbs.
You should consider a custom shirt order that lets the customer pick one of the text styles and select their own words. Then someone would be able to order, say, the Wars of Stars font with "Jed-eye" for the father and "Paddy-wan" for the son... (*cough cough trademarks*), or even the "BIG MAN" font with "BIG BOB" and "LITTLE BOB" for a father and son pair who share the same name.