In general it's recommended to wait until the birds are similar in size - basically meaning the chicks should beat least 6-8 weeks old. Even then they will be the new guys in the enclosure and thus likely to be bullied, but the fact that there are several of them and that they are already living in the enclosure so they are used to seeing each other will probably help.
You could try placing one of the older birds with the chicks when they are around 2 weeks old and a little more hardy as CR suggested, just to get an idea of how the older birds react to the chicks. If it doesn't seem aggressive you could try with another, then maybe 2 at a time. I find it highly unlikely that it will work and I wouldn't leave them alone for a minute unless I'm absolutely positive nothing will happen, but if everything is still fine you could place a few chicks with the older birds and see what happens then.
Also, you are going to need to separate the males soon, or they will start fighting (usually happens when they are around 6 weeks old). If you remove the big males just before introducing the chicks, you might have messed up their pecking order enough to allow the chicks to just blend in.
Keep in mind though that I've never had coturnix and most of what I know about them is from other posters on this forum.