So you've grown something with very little natural light up to a foot high. That is going to be tricky. Those tree's are full sun varieties so by growing them in shade it's very likely they have become weak and leggy searching for light. Also did you take the cuttings from above the graft of the original trees? If not, then you may actually get a different variety of tree depending upon what the grafting stock was.

I would harden them off this way.

1. Leave in existing location but remove the plastic bags. In order to do this successfully you will have to feed and baby these plants a bit. They are used to constant humidity and water. So once you remove the covers immediately feed with a 1/2 strength seaweed solution. For the first week use a handheld mister to keep the humidity up by misting them twice per day. Then the following week, once per day. Then they should be used to the ambient humidity. DO not allow their pots to dry out in this first week.
2. After the first week of no coverings, start rotating them out into the light for a few hours each day. NOT direct sun to begin with. Choose a dappled light location under a large tree and only put them there for 2hrs per day first week. Then 4hrs per day the 2nd week. Watch them carefully they should develop a darker colouring to the leaves and should not drop leaves if they are happy. Within 4 weeks you should be getting them out into direct sun for a couple of hours per day and in a sheltered outdoor location at other times.
3. You also have to harden them off to deep infrequent watering. In the first two weeks do the misting but also make sure you water whenever their soil is dry. You want to keep that moist. By the third week you need to water less often until you are only watering once every 2 days, and then finally once per week. Citrus do best when watering deeply and infrequently. But keep in mind the pot size will change this somewhat. Being in the ground keeps their roots cool and moist but plastic pots heat up quickly and dry out quickly. The larger the pot the more moisture it will hold. So leave them in their current pots while you habituate them to the outdoors then once they have stabilised (4 weeks or thereabouts) transplant them into larger pots to enable their roots to expand and the water schedule to ease up. For a small tree a 27cm pot is really about the smallest size you will want to consider and once transplanted into this size you will want to leave them a full year to grow in and become strong and healthy trees with good roots. My full size fruit trees are in 60cm pots.

If you find the pots get too hot or too dry for the plants there's a trick you can use. Dig the pots into a hole in the ground leaving only the lip above the soil. It makes your tree removable without damage, contains their roots but also gives them the benefit of the cooling insulation of the ground.

Thank you, very helpful. I omitted to mention they were on garage windowsills so they did get natural light.

Even so, indirect light like that will make them tender. But persevere they will probably come good. A relative of mine grew a lemon tree in full shade by tossing a seed out the window. Not only does this wild lemon grow without any direct sun but produces bunches and bunches of lemon all year round. It's constantly in flower. Sometimes doing something different works.

@Black Bamboo Thx for the advice. I'll try to do the same next year. Probably, 20 cm is too small to get them outside? And how the type of greenhouse can affect growth and growth rates? Thinking about smth small, like that

Thanks in advance

Citrus cuttings are a great way to increase the productivity of your citrus trees. By taking cuttings from healthy branches, you can help to propagate new plants and keep your trees healthy and thriving. Cutatures should be taken during the winter when the tree is in dormancy, and growth will occur rapidly over the next several weeks. Once rooted, Citrus cuttings can be moved up into sunlight or a warmer location if desired.