It’s certainly grave matter. If done with full knowledge and deliberate consent, it would be a mortal sin.
If you married with the permission of the Church, you were required to promise that you would maintain your Catholic faith and do your best to raise your children in the Catholic faith. It appears that not only is your child attending a Protestant church, but your own commitment to maintain your Catholic faith may be slipping out of a desire to please your spouse. Perhaps this situation demonstrates why the Church strongly disapproves of interfaith marriages. While some couples are heroically able to make them work, all too many Catholics involved in such marriages often are not able to raise their children Catholic and put their own faith at risk as well.
I can only encourage you to examine this situation more closely and to pray for the grace to realize that following Jesus Christ in the Catholic Church, which he founded for your salvation, is more important even than pleasing your spouse by attending a non-Catholic church “as a family.”