How do you work on the 'spark' once it's already gone? Any tips on how to revive it?

I would think your priorities could have been shifted. Previously before having kids, it's just the two of you enjoying each other's company. But with kids, they have become you and your husband's priorities. More time are spent on the kids than before. Both of you may want to find time every night to have little chat after your kids are asleep? Or finding time for "dates" on weekends. Otherwise, you can take half day off to go for nice lunch or movie together with your husband if it's not easy to do "dates" on weekends. Communication is a pretty vital part of a relationship. All the best! :)