Spanish is the easiest language to learn, even easier than English. I took Spanish in High School and the teacher explained why this is. Our language has a lot of words that sound the same but mean different things, words that are spelled differently and sound the same. It can be extremely confusing for children and even many adults. This is not the case with Spanish. Also, the letters of the alphabet each have their own sound and are always pronounced that way, as opposed to English where we have long and short vowels; hard and soft consonants. The younger a child is when they learn a second language, the better able they are to learn a third or fourth language. Since Spanish is a Romance language it is a gateway to other languages, such as Italian or Portuguese. Learning Spanish can also help children to understand the origins of words, which in turn can teach them to do better in other classes, like History and English!
If you are interested in helping your child to remember and retain the new language more easily I can highly recommend a book by Harry Lorayne called "The Memory Book: The Classic Guide to Improving Your Memory at Work, at School, and at Play". My High School Earth Science teacher used this book for six weeks at the beginning of the school year to help us all improve our grades. It has a section on Foreign and English Languages which helped me to retain more of what I learned that year, not only in Spanish, but in English as well. I hope this helps you make your decision.