some(most)parts are oversized.Most tandems use oversized headset--not hard to find if a somewhat modern tandem.They used mountain bike headsets.Early english tandems like the russ I did used built in cups which would be a pain to find(the russ used Chater lea headset and BB which is a special CL size)Glad it came with them!
For rims tandem use 40 or 48 spoke rims and tandem rims are usualy a bit wider than regular rims of the same make.These are easy to get---sun as an example makes lots of 40 hole rims.Hubs are wider on tandems too so try and get a good set.A rear hub with threads on the left side for a drum brake is a plus.
All in All,if you find a good used tandem like the one you showed rims are an easy up-grade.
Seat tubing may be oversized ---shims can be used to use regular size seat posts. Derailers need to be for oversized tubes,but regular stuff---M/B rears work good for larger cogs.Tandems don't pull as good on hills so larger rear gears are good.
All the above is only about the mach. of putting a tandem together---the real question that needs to be answered is:
Will your son or daughter be able to ride with you/will you be able to ride with them?Remember you must be in sink with them while ridding.And also the tandem must fit not only you but them also.---sam