No, the price of the course set menu has nothing the do with the value of the voucher, it's always best value to order the set menu (for your voucher type, you can also order cheaper ones without paying) because no matter what, that will be free. But the value of the voucher is the amount you paid for said voucher. Because breakfast was included in the hotel stay, it was easier before, the value of the lunch/dinner voucher was the cost to add one night of half board to your booking. But because breakfast isn't included in the hotel stay, but it is in the half board, now the lunch/dinner vouchers are less then that price (because the price for half board went up to include the breakfast, the vouchers probably don't have less value). So if it's 30 euro's to book one night of half board, the voucher might be worth 25 euro's. Even if the set menu is worth 40 euro's, you'd still have to pay 17 euro's for the 42 euro a la carte item.
I don't know the value of the vouchers, the 25 euro is just a random number, it's printed on the voucher (I also don't know if it's always the same, because I heard rumors it wasn't). We don't book half board, currently we have annual passes and the discount on that works better for us. We went without an annual pass (in between passes), I looked at which restaurants we wanted to eat (mostly buffets, so that factored in to it) and calculated the price and it would be (a little) more the get the vouchers (we were staying at club level, so didn't need the breakfast).
For the childrens menu, as with the story above, if the adult plus menu is 40 euro's and the children's premium is 38, if the value of the voucher is less, you'd still have to pay