The exact position for sleeping depends on the severity of breathing issues and how they are managed. Advice should be sort from your specialist nurse. In general all Pierre Robin babies should be nursed on their side for the first few months. 

When travelling it is recommended that the baby seat is positioned as upright as possible, however if your baby has breathing difficulties which are corrected by poitioning the baby they may need to travel in a carry cot so that they can lay on their side.