It's not necessary at pre-K. But it doesn't hurt to go to a camp. Ask at your school for suggestions.
For 3-6 year olds with no Spanish, CommuniKids is great introduction. They only work with young children and it is truly immersion. They understand parents' concerns about language learning and are very supportive.
http://www.communikids.com/our-programs-/summer-programs
WIS is OK for slightly older kids or young ones who have some Spanish. But it's a much bigger camp with other languages and English only programs. Not exactly a full immersion environment.
I have no first-hand experience with Tyler/DPR, but have heard from others that it is designed for young children entering bilingual for the first time.
HTH