First time foster parent looking for advice

I am a first time foster parent and I just got a brother and a sister. The sister is 8 and the brother is 14. They were both really quiet coming into my home. I showed them both their rooms. They're trying to adjust the best they can. In the brothers room I noticed a funky smell and while he was at school I went looking around and found he had wet the bed and hid the sheets under the bed and his mattress had pee on it. I went to check the sisters room too and it looks like her bed was dry. I didn't know if it was a one time thing so when he went to school again I checked and he has wet his bed. How can I approach the situation with compassion and sensitivity and keep from embarrassing him?