Mudguard (fenders) and racks for your bike should be available at any local bike shop. They should be able to offer a serviceable rear wheel, too. As @
LV2TNDM mentioned, upgrading to a cassette hub would be worthwhile.
There are a couple of shops in the UK with a decent selection of tandem-specific hardware. You're going to want a child stoker kit at minimum. That should include an add-on bottom bracket and chainset (crankset) that bolt to your seat tube. Here are some potentially useful links.
Explanation of what to do:
PRECISION TANDEMS - Child Stoker Kit Installation Guide, child stoker, kid-back, kidback, kid kit, tandem, riding tandem with children,
Tandems East Cranks & Pedals
Parts in the UK:
https://www.tandems.co.uk/opening-times/
Circe cycles accessories child | Circe Cycles
https://www.sjscycles.co.uk/componen...black/?geoc=US
https://www.sjscycles.co.uk/componen...eners/?geoc=US
https://www.sjscycles.co.uk/cranks/s...ilver/?geoc=US