At this point they don’t have to do the measuring perfectly. The most important is to get the idea behind it. You can either use formal (rulers, measuring tape, measuring jug etc.) or informal (socks, stones, cubes) units in measurements.
Try show them how to use it and watch them experimenting. I like to put measuring tapes in a construction area in my classroom and watch the kids measuring how big is their build.
Keep a measuring tape in your bag. Take it out when your are out for a walk and feed your child curiosity by checking how big/or long are things around us. Don’t give your child a diy measuring tape without your supervision, as they can cut their little fingers. Use sewing measuring tape instead. Oh, I always bring a tape back from ikea. It is a pity to dump it after use.
This set is amazing to start your little lab. I don't recommend to buy coloured test tubes, because it is better to use food colouring in the water. Your child will have way more fun mixing liquids and colours when they can see them well. I have few coloured (that I thought are very cute) but non of my kids like them...
Click on the picture to view it on amazon.