Here are some ways I help my children build their Spanish vocabulary. I know that I have an advantage since I am fluent in Spanish but I also help my 4 year old in other languages he is interested in.
1) The Internet is your friend! My son loves languages and lately he is constantly asking me how to say this or that in French or Sign Language. He has become interested in French because I mentioned I knew the numbers in French and he is interested in Sign Language because he signs in a church choir that teaches them the songs in Sign Language. So, any time he asks me how to say something in French or Sign it, I go to the Internet. Here are some free websites my children and I use for Spanish.
2) We listen to Bilingual and All Spanish Children's Music in the car. I have several I recommend here plus I personally have some just in Spanish since I can translate for my kids. It is important that if you do not know Spanish that the Cd's be Bilingual so you can help re-enforce or explain to little ones. My kids are really into the Teach Me Spanish Music CD right now.
3) If we meet someone that speaks Spanish, maybe someone that works at a restaurant or grocery store, I encourage my children to speak Spanish or ask the person how you say some word in Spanish or converse with the person in Spanish. If you are at a restaurant and the waiter speaks Spanish, you can practice ordering in Spanish (ask the waiter to help you). If you are going grocery shopping, make part of your shopping list in Spanish. For non-readers, ask them for the food in Spanish. "Where are the apples? I need two apples."
4) We set aside a specific time in the day to practice Spanish. With small children, bath time is a great time to practice the body parts and all of the vocabulary that goes with taking a bath. You can practice foods at meal time, grocery shopping, and making lunches. Numbers, Days of the Week, Months and Weather with the calendar, numbers with math activities, colors when playing, clothing when dressing in the morning, etc..
I personally want my children to be completely fluent in Spanish so I will be taking them to Puerto Rico (my home) in the summers and enrolling them in summer programs there so they will be immersed in the language.