Kids clothing sizes

I have a big parenting blindspot when it comes to my kids' clothing sizes. My wife buys all of their clothes, mostly from thrift stores, and is always on top of buying the next size up.

For the most part we can do a "divide and conquer" approach to parenting responsibilities, so it's not a huge issue between us. However, a friend was asking me about the kids' sizes for a surprise Christmas gift, and I feel like a total chump for not knowing.

To be totally honest I barely know clothing sizes for myself-- I don't know my wife's clothing sizes or the kids.

Any tips on how to get better at this?