Bavaria travelling

I'd reserve them online in advance just to be sure.

Yes to what Lee said -- you don't want to spend hours on the train and then find the castle tickets sold out (which CAN happen, especially on weekends.)

We did the same trip last fall. I would definitely get the tickets ahead of time,

Online for sure. We were there this past Wednesday and the reserved line was about 10 minutes wait while the purchase on-site line was huge. If you are coming from Munich by train or car? The cheaper Bayern train ticket won't get you there by 11.

My wife and I and 2 grandchildren (15 & 16) will be going from Munich to Fussen on the 17 (Sun) of June. If we arrive in Fussen at 11:00 AM,

Please note that due to construction work train transport in the morning of Jun 17 is limitied to the train leaving Munich at 8:40. You will arrive within your planned time frame but there would be no alternative (when looking up connections at www.bahn.com click on the small red triangles next to departure times and read what they mean).

"...you don't want to spend hours on the train..."

The normal round trip from Munich to the final Neuschwanstein Palace stop involves train but also a bus - and of course the same going back to Munich; it's nearly 5 hours of transport time. Once there, you have a fairly steep uphill walk to the starting point for tours - plan on another hour's walk + for the round-trip. Grandparents of teens should be aware of the caution at the N'stein website:

On foot... It takes about 30-40 minutes to walk from the ticket centre
to the castle (around 1.5 km on a steep uphill road).

You can spring for a carriage ride instead. But you won't get the whole way:

Because of space limitations, the carriages cannot go right up to the
castle. From the carriage turning area beneath the castle it is an
uphill walk of about 450 metres or 5-10 minutes to the castle
entrance... Reservations are unfortunately not possible.

"...due to construction work train transport in the morning of Jun 17 is limitied to the train leaving Munich at 8:40."

"...definitely get the tickets ahead of time..."

Also from the website:
"...tours last approximately 30 minutes."

In light of all this it wouldn't surprise me if you were interested in alternatives.

Nymphenburg Palace, where mad King Ludwig was born, is right in Munich, has much more to see, and is easily reached by local public transport from anywhere in Munich. You could spend a few hours there and still have time for other things in Munich.

http://www.theworldisabook.com/16991/munich-with-kids-nymphenburg-palace/

On the grounds there are several choices for a mid-day meal:
http://www.schloss-nymphenburg.de/englisch/tourist/restaur.htm

Please note that due to construction work train transport in the morning of Jun 17
is limited to the train leaving Munich at 8:40.

I've seem this before, where the Bahn first announces that the train is cancelled, but still shows the connection and an arrival time at the destination, then, before the date, announces a schedule with an SEV (Schienen Ersatz Verkehr = replacement bus). However, given that the earlier train at 8:40 is only 13 minutes earlier and gets you to the bus stop in Hohenschwangau at 10:43, just plan on taking it.

In light of all this it wouldn't surprise me if you were interested in alternatives.

Schloss Nymphenburg is interesting, and I would recommend it if you have time, but never as an alternative to Neuschwanstein. There is only one Neuschwanstein; there is no alternative. It is such an icon of Bavaria. I went there once, in 1988, and have come back twice, both times to show it to a friend, a newbie in Germany, because I thought is was so important.

The walk up to the castle is not that bad; I last did it at 58. The last time, due of my partner's recent hospitalization, we took the carriage. True, it doesn't go cleart to the top, but it turns around at a little restaurant. We had plenty of time before our tour, so we had lunch before continuing on, then stopped half-way to the castle to wait until our tour time was imminent.