"as a result" - this phrase is used to introduce the consequence or outcome of a previous action or situation. in the context of the email, it's used to show that the experience gained from helping out in the local church summer school for children is a consequence of the previous statement about being interested in working with children.
"therefore" - this linker is used to indicate a logical consequence or conclusion based on what was previously mentioned. in the context of the email, it's used to connect the statement about the applicant's involvement in athletics and tennis to the assertion that they feel particularly suitable for the position.