Here is what we do.
I ask my husband to do something for me that needs to be done but I don't want to do. My favorite to ask is detailing my car. I ask him to do this every Valentines and Mothers Day. It's cheap but yet sooooo nice. I usually give my hubby a full body massage. My husband doesn't get these on a regular basis even though I am a massage therapist. You know your partner better than anyone else do something that they will really appreciate to show your love. Cause that is what that day is all about yeah?
If your partner is the kind that likes receiving gifts more than not, then find something that they need or something cheap that they wouldn't already buy for themselves. For example, when my husband is feeling like a big spender he will buy me my favorite bottle of wine, I'm cheap it's 11 bucks, and chocolate covered strawberries. I love it!!
Kids are easy, chocolate! I buy them whatever cute chocolate/candy box that they are into for that year. For dinner, we cook, or go to Papa Murphy's and get the heart shaped pizza. If I remember right its between 8-11 bucks a pizza. They love it.
After we send the kids to bed, we get romantic with Netflix and Chill.
For the whole day, I can spend as little as $40, including the pizza and kids gifts. We like it, it's a low key low spend holiday for us. I get my car cleaned he gets some much-needed attention and the kids have fun eating pizza and chocolate.