My wife type2 dm past 6 years &having 5 yr old child.hba1c<7.shall we have 2nd child?

Why not: You wife's condition as such should not prevent you from having the 2nd child. Her sugar should be well controlled before conception and during the entire pregnancy. Control should be tighter than usual: target a1c 6%. It is important to have regular obstetric and medical follow-ups to ensure best possible pregnancy. If you have specific concerns, you should visit your doc to help plan and prepare.