It has been a while since I have stayed at the Dolphin and it is beautiful, but you got a great deal at the Poly. If the points will keep, you could save them and use them another time for another trip when things may be really tight and you need a vacation.
The Poly is amazing with children. The kids will be able to see the castle with the fireworks from the pool, so you wouldn't have to be in the mix. You can ride the monorail or take a boat. Monorail to Epcot as well. The food is awesome and kid friendly. Tiki lights at night are romantic. Music played throughout and Disney surrounds you.
I have offered to stay luxury now that my kids are older just to try, but staying at the Poly was so magical our first time that they dont want to stay anywhere else. It has been hard convincing them to try something new, especially if the budget calls for it. The Poly is our home resort.
Good luck! Great problem to have.