when a budgie is very young (as in, before you got it), it's very important to offer the birds a mixed diet so that young birds are introduced to lots of different new foods when they are most likely to try them.
If the breeder only offers the bird basic foods, once they are ready for rehoming, they will be more likely to just stick with what they know.
The only thing you can do is keep offering them different budgie safe foods and one day they may try them.
It's very important to remember that just because your budgie doesn't want to try and of those foods today, it doesn't mean that they wont eventually try them.
Romaine lettuce or other leaf veg, such as spinach, usually is an good food option for your birds (although don't keep offering spinach as too much is bad for your birds) to get them started. It's all about finding out how your bird wants to take the food, so try and mix it up.
Sometimes on a dish on the floor and wet, sometimes poked between the bars of maybe torn into small piece in a regular food pot. Keep trying different methods to get your birds interested and eventually they will try other foods.
Also, if you give them millet spray in their cage as well as their regular seed, cute out the millet spray as this is a 'treat' and might help encourage the birds to not fill themselves up on their favourite treats.