The diet should include very little seed actually.. Pellets are important, as are fruits and veg. Vegetables are more important than fruit, as too much fruit can lead to diabetes and obesity (as well as hyperactivity).
Fruit can be a good treat---bits of grapes (not whole ones), bananas etc.
Plain oatmeal can be fine in moderation---same with bits of boiled egg and certain types of seeds/nuts (but cockatoos are prone to obesity so you have to be careful).
Cockatoos are THE MOST RE-HOMED bird of all time (and they live for a long time)--among those re-homed, Umbrellas and Moluccans are at the top of the list (but all types are re-homed disproportionately to other birds). Rescues are over-flowing and they are
extremely challenging birds to care for --hanging out with one is like 1/25th of owning one and being responsible for all of its daily feeding/cleaning/chewing/veterinary/social needs. If you get one, you would be wise to adopt an adult so that you don't have to worry about spoiling it etc (babies are very easy to spoil, but with an adult who has gone through puberty, you will be better able to see his/her true personality). Babies change a lot and there is such a surplus of cockatoos that need homes...Please adopt instead of shopping. There a zillions of these sad birds filling up rescues....but remember....they are surrendered frequently due to the fact that they are a poor fit for 95% of people who select them.
Many people are drawn to the fact that these birds are known for their "cuddles" but cuddles are toxic for a cockatoo. They are purely sexual and lead to all sorts of problem behaviors--- so all of these pictures of people holding them like babies and snuggling them against their chests=very bad news for the bird long-term. A cockatoo should NEVER be touched anywhere other than the head and neck---no under the wings, no back, nowhere but the head and neck....and they should also not have access to any shadowy spaces (e.g., around pillows, blankets, under furniture, in boxes, tents, huts, under clothing etc). Exposure to these types of places triggers nesting behaviors and misdirected sexuality which leads to amplified behavioral problems (and even potential health problems, such as egg-binding) in an already behaviorally-challenging bird.
There is a lot of potential for things to go very wrong with a cockatoo, so I am not sure what the extent of your experience is, but I would highly recommend volunteering at an avian rescue (long term) so that you get a taste of what they can be like in larger doses. The fact that you have never seen your parents giving one treats implies that the time spent was in short stints and that you have never been the sole care-taker... If you have to wait a few years in order to volunteer and get to a more stable place in life, a few years is a drop in the bucket compared to the 70+ years that your bird would be with you. Also, since they live forever, please consider how this may impact your life long-term (housing, noise, babies, other pets, finances etc). They are an absolute no-no for apartments.
They are completely unique from other large parrots...As much as I love mine, there is no one in my life right now (not even my best friend) who could (or would want to try to) successfully live with a cockatoo (due to the sheer amount of work it takes--you pretty much have to be insane to take it on). If you get one, think if it as marrying a perpetual 3 year-old with off-the-charts ADHD, wings, a built-in chainsaw, the volume capacity of a jet engine and a terribly fragile respiratory system...Who may or may not accept the other people in your life...Their tri-point beaks
can snap a child's finger off and an aggressive or jealous cockatoo can do hospital-worthy damage to an adult.
They need consistent daily bedtimes and wake-up times; they need meal-times. They also require more sleep and out of cage time than most parrots...
We are talking 4+ hours out of the cage daily --minimum (with 12-14 hours of sleep each night--for other parrot varieties, 10-12 hours is generally the necessary range, but
not with cockatoos). Sleep is non-negotiable and it has to be on-schedule, as it regulates their hormones, immune health and behavior. Think about how this will work if you have a typical work schedule...Even if you get up at 4AM, that means your bird will have to go to bed at 4PM, which leaves insufficient time for interaction if you have to go to work at 7---and if you don't get off work until 6, your bird is going to be up until 6--which would result in a sleep deficit (and subsequent problems).
It is not all bad obviously, but the re-homing rate speaks volumes about what people think they can handle, compared to reality. Time and time again, people research and adopt these birds, only to find that they cannot manage any longer. I am not saying that is you, but no one ever thinks it will be them, or they wouldn't bring one home to begin with.
There are some people who will speak only about the positives, and while there are positives, the average person's experience is obviously quite the opposite or there wouldn't be a surplus of unwanted cockatoos. SO, again, if you get one, please adopt, but before you do that, please consider everything I mentioned above.
http://mtrushmorebirds.com/cockatoos-think-carefully-before-buying/--as you read this, remember, while cockatoos are "known for their cuddles" (and seek them out) they should not get them. Cuddles are very unhealthy for these cuddle-seekers.