Not a great experience with MTD on Symphony. Long wait with or without reservations. Not well managed, tables were very uncomfortably close.
I would suggest what a previous poster did : early dining with a fixed table and dedicated wait staff so your all of your needs are met. 5.30 pm does not mean that your meal will be served on the dot of 5.30, the doors open at 5.30, so it may work out fine in the 6 ish range.
Enjoy your cruise. I hope the kids are thrilled! Ship has a lot to offer them.