Hi Heather!
Frankly, with only 4-5 days max for the islands, you can't do Rhodes AND Santorini -- They are waaaay far apart. Did you look at a map?? Even if you flew, you'd have to Fly from Rhodes back to athens and then fly OUT to Santorini. In late Spring thru late fall, there's an overnight ferry from Santorini that arrives 7:30 am Rhodes... but in March the only ferry stops at 10 places, takes 12+ hours. Often we find people indulging in Magical Thinking about instant transport -- kind of a "Beam me Up, Scotty!" wish. Maybe in Star trek, not here! Anyway, Santorini is really all about the Caldera View, for kids nothing to do -- Rhodes MUCH better. So your options are NOT overwhelming. Save Santo for next time!
Rhodes - stay in Rhodes Old Town; it has a considerable year-round population I particularly recommend staying in a modest little place -- http://www.medievalinn.com/# -- because it's in one of those old lanes right against the Medieval wall. No 2-room suites, but your kids are old enough to stay in a room adjoining yours. Ask for a room on upper floor, the street-level rooms are dark.. there's a terrace up top.
Here are some very useful websites to use in preparing for 3-4 days on Rhodes:
GOOD Island map - http://www.faliraki-info.com/rodosmap.htm
BEST Old Town Map (print) http://www.planetware.com/map/rhodes-map-gr-rho.htm
RHODES TOWN SKYMAP - http://www.skymap.dk/map.asp?ElementId=59319&Category=0
PHOTO WALk - https://tickettoadventures.wordpress.com/2016/10/26/walking-medieval-rhodes-old-town-moat/
-- at bottom of this, 4 more links to Rhodes Town & island sightseeing with PIX!
My own Rhodes tips: • Do NOT miss the walk atop the walls! Ask Hotel about when; it may differ off-season • A walk out to the sea and Ancient Rhodes temple & Stadium is a kick... AND authentic • Parts of Old Town and of Lindos are fake-restored by Mussolini's minions, but fun anyway, and the walls are totally real!. To make it more exciting, get them a historical book The Siege of Rhodes, or a novel Siege of Heaven (which glamorizes the crusaders more)... either one used $7+ on amazon. I read a novel on the siege, badly written, but made Rhodes MUCH more interesting for me.
To minimize travel time, maximize enjoyment time, avoid going into Athens beginning AND end of trip... it eats up precious hours to taxi from airport-checkin/check out TWO times. Besides coming from Canada u may be tired, and kids won't want to leap out of bed to see Acropolis. Thus, stay in Athens LAST. On arriving, stay in airport and ASAP (allow 90-mins between planes) take domestic flight to RHODES. I guessed at Sunday March 10, and flights all day are €57.94 (change that to loonies), and going back Wed or Thurs even more of a bargain, €36.55... these are far less than ferries to/from. This is the "flex" price, includes luggage, & is changeable without penalty. Here's the link -- https://en.aegeanair.com/plan/book-a-flight/ -- as soon as your Greece dates are firm, I suggest booking these flights. A bonus: "Learning the ropes" about how things work in Greece can be done at an easier pace on an island, so when you return to ATHENS, your family will be revved up and ready to make the most of a bustling capital city with AMAZING landmarks and history.
