I have success feeding my parrotlets organics baby food (e.g.: mixed veg, chicken'n'yams, strained peas, etc.) Heinz or Gerber doesn't matter as long as it's organic. If you can, melt a little red palm oil and mix it in (they really like it that way). You can also mix in some soaked seed (soaked for 8 hours in warm water, then thoroughly rinsed.)

Also, try offering small pieces of fresh tropical fruit like bananas, guava, papaya, mango, pineapple, etc., and domestic fruit/berries such as strawberries, blueberries, pears, kiwi fruit, etc. Fresh greens like kale, Swiss chard, Romaine lettuce, beet tops, spinach (in limited quantity) are also good to offer. I recommend that you buy organics, if at all possible.

Don't offer too much food at any one time, they tend to waste it by tossing it around. If the portions are small, they are a lot more careful, just make sure that you change out fresh foods frequently. During warm weather, fresh food should be removed after an hour, a couple of hours during cooler days.

Have fun and feel free to experiment.

BTW, foods/liquids to avoid (for cockatiels in particular):

Avocado
Brazil nuts
Chocolate or cocoa beans
Coffee (beverage or beans)
Alcohol & carbonated beverages (including beer & wine)

Always be careful about offering unknowns to your birds, if in doubt DON'T!