Hi Lena,
It seems that your child has hives / urticaria (skin rashes due to allergy).
You are right, antihistamine medicines [
loratadine, cetirizine,
levocetirizine, fexofenadine] are used treat such condition.
You can do the following to manage itchiness.
1. Use loose clothings that don't compress the skin.
2. Keep her cool, avoid sunlight and laborious works / exercise.
3. Applying Calamine lotion over the skin may decrease the itchiness.
4. If you can suspect / find the allergen (the agent causing allergy by inhalation, ingestion, contact etc. ) - avoid contact with it.
Visit your doctor if the rashes do not come down, or increase and to get the diagnosis.
However, if you see any danger signs [ feeling of a lump in throat,
dizziness, noisy respiration, troubled breathing, fainting], this signify severe
allergic reaction (
anaphylaxis). Visit the hospital emergency immediately.