I have two lemon trees and I bought it two big pots. What's the proper soil for the seedlings, as I kept a little sun on the balcony of their house.
First thing to consider is the size of the pots for your lemon trees. You don't want to put the tree into a pot that is too large, because the soil tends to stay too wet and roots will rot. The pot should ideally be only a couple of inches (pots are measured by the top diameter)larger than the pot in which the tree has been growing; usually no more than 4 inches larger. You mention seedlings - if you really have seedlings, those would be in very small pots, probably no larger than 4". You will need to grow them on into 6" pots, then 8", 10", then you might go to 14". Now, you may not have seedlings, but small trees, which are usually grown in 10" or 12" pots. In this case, you could put them into a 14" or even 18" pots.
For the soil, you should use a commercially mixed potting soil without "moisture retentive" additives. You want something with good drainage - cactus soil is ideal. Also make sure there are drainage holes in the bottom of your pots, because the best soil in the world will not work if the water doesn't drain.
"A little sun" may not be enough, lemon trees are going to need at least 4 - 6 hours of sun daily, and more if you can. This article will get you started on a successful experience with your lemon trees: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm