Babies are constipated! Please help!

My twins are 11 weeks old. They were born at 36 weeks so there’s some “preemie” adjustment I believe but they have been formula fed since 2 weeks old. We haven’t changed their formula or anything to their “diet” and both of them haven’t pooped in 2 days. They usually go once a day. They act like they are trying to get it out but nothing comes out except occasional gas. Are they too young for a pedi lax or do I keep my fingers crossed they go before day 3 of no pooping?