What's the best way to tackle a relative who has an opinion on everything related to the way I bring up my children?

You will encounter such people almost at every step of your parenting journey and the best way to deal with such people is hear them out but do what you feel is best for your kid. This way, you do not upset or hurt their sentiments and still get to do all things parenting. Plus, you never know, you might get actually get some good advice or get to know about something you didn't know before.