I second the Signing Time suggestion. It's a GREAT resource. It's not a website per say but rather a show/DVD's that you can play. You can view parts of some on YouTube to check them out but would have to pay for the rest but it's definitely a worthy investment. And you can buy them slowly over time since they start with beginner words and work up from there as the videos progress. My kids all love them. The Aunt Rachel for the show does her own thing also and those videos are great too.
Rachel's TreeSchool:
https://www.youtube.com/channel/UC2q...iuPmH6w/videos
I also like Baby Sign Language. They have videos to show you the motions of the words, written explanations and clipart that you can print and use as flashcards to post.
https://www.babysignlanguage.com/
As far as teaching them Spanish ... you mention not knowing it well so I'd like to share something for you to ponder. Pronunciation and phonetics is important so is proper usage of words when learning a language.
Mispronouncing words can unintentionally teach the language incorrectly which can make it difficult for them later if they try to learn the language at a later time. Essentially they'd have to un-learn what they learned incorrectly and then learn it all over again. So use Google Translate and other websites when you can that let you listen to the way the words are pronounced and practice yourself before you teach the new word to the kids and they'll do great. Google translate has the "Listen" option (looks like a speaker/volume icon).
First make a list of all of the everyday words that you use the most during a normal day and then look up how to say them in sign and in spanish. Start with 10-20 common words and then add more as they start understanding.
Words like Mom, Dad, Eat, Sleep, Change, Hungry, Play, Help, Thirsty, Drink, and common snacks and food you eat are a great place to start.
I took a workshop about teaching sign to babies and pretty much you can start as early as 6 months (to introduce words) but they won't start to repeat words or hand gestures until between 9-12 months.