A nice place to look for ideas is on Christopher Danielson's website, Talking Math with Your Kids. For kids that young, one of the most important things is not to rush it; make sure the kid is having fun, and has a rich environment to explore. The hashtag #tmwyk (tell me what you know) is a good one to see how people listen to kids. And listening and playing are the first steps for sure.
I do sometimes use two questions to help dig a little deeper into mathematical thinking with young kids, and they are "How many?" and "What if?" How many is really one of several questions that can draw kids attention to ideas of number and magnitude. How many blocks are there? Which is bigger, that tower or that tower? Who is smaller, me or you? There's a lot of richness there.
What if is even more flexible. There's an opening to experiment here: as in, what if we tried to build that shape using only the red blocks? Could we do it? What if I moved as quickly as I can? Could I go faster than you?
You can also narrate your own thoughts out loud as you try out these questions yourself. The key is to avoid being pedantic and explaining. Set yourself up as a listener. And avoid asking yourself, "Is my child where they need to be?" Instead, ask, "How is my child thinking?"