In reply to kipper12:
I wouldn't worry too much. The only problem feeding young are whole peanuts as these can, and do, sometimes choke young birds. Some people put them on the ground and not in mesh cages where the birds must take only small bits.
I've been feeding birds and watching them for many years and the only other possible problem is from dirty feed trays which may carry some avian infections.
I've been reading bird magazines for many years, including the RSPB and cannot recal any concerns regarding suet.
If you are concerned about a varied diet though, you could try putting the suet out for just a morning or afternoon, which will result in the birds having to find alternative food sources. But I really guess that although it may appear that the adults are only sourcing your suet they are also likely to be fetching other food from time to time.