Wananipika? = Are they cooking me? [I am the food]

Wananipikia? = Are they cooking for me? / Are they cooking me something?

verb conjugation (mnyambuliko wa vitenzi) . In this case the verb 'pika' is conjugated to the form of kutendea/kufanyia, i.e. doing an action for/on behalf of someone. So the suffix '-a' becomes '-ia/-ea', depending on the verb

Pika - pikia

Cheza - chezea

It will depend on the noun class of the object .e.g. onion (kitunguu, noun class 'Ki/Vi') as an indirect object would be: 'Wananikipikia?'.

As a direct object, it depends on whether you're speaking generally or referring to a specific onion ('a/an' versus 'the'):

Are they cooking an onion for me? - Wananipikia kitunguu?

Are they cooking the onion for me? - Wananikipikia kitunguu?