Those Christian seders are interesting in their symbolism. But there are problems with Christians celebrating a Jewish ritual. Not only are there Christian reasons not to celebrate the Passover, but there are also Jewish reasons.
This is explained by two Jewish rabbis writing in Christianity Today. Their fascinating article shows an impressive understanding of both Christian and Jewish theology. They point out that Jesus did not, in fact, eat a Seder meal. He ate the Passover, but not the ritual as practiced by Jews and now some Christians today, which was started long after the destruction of the Temple. They also explain why it is disrespectful for one religion to take over the rituals of another.
Their argument is sort of a Jewish version of what Lutherans take heat for in their practice of “closed Communion,” that those who commune together should be unified in their ecclesiastical community and in their confession of faith. Call this “closed Passover.”