1yr old twins. Have blotchy rash. Same soap/lotion since birth. Can they now be allergic now? No food changes. Using hydrocortisone cream

Yes. The children may have atopic dermatitis and even though there has been no food changes, food allergy can be acquired through direct skin contact if AD is the issue. Talk to your doctor and consider seeing an allergist.
Many causes of rash. Very hard to tell what a rash is without seeing. Viruses can cause rashes. Contact with an irritant. Could be hives. I see they are 12 months olds. Did they recently have the MMR & Varivax? Both can give rashes. The only way to know is to have their doctor look at it.