Children should wear الإحرام garments with leniency applied — the parent makes niyyah for young children, and no فدية is required if children violate الإحرام restrictions.
Children can and should wear الإحرام garments when performing الحج or العمرة, following the general rules with appropriate leniency for their age. For boys, this means the two white unstitched garments (izar and rida) similar to adult men. Small الإحرام garments sized for children are readily available in shops near the Haram. Girls wear their normal modest clothing, as women's الإحرام does not require specific garments.
The parent or guardian makes the intention (niyyah) on behalf of a young child who cannot understand the concept. For older children who understand, they should be encouraged to make the intention themselves with parental guidance. Regarding الإحرام restrictions: if a child covers their head (boys), uses scented products, or otherwise violates الإحرام restrictions, no فدية (penalty) is required عند جمهور العلماء. This is based on the hadith: 'The pen is lifted from three: the sleeper until they awake, the child until they reach puberty, and the insane until they recover' (Abu Dawud). However, parents should still teach children proper الإحرام etiquette to the extent their age allows.
Source: Sunan Abu Dawud; al-Nawawi, al-Majmu'; ابن قدامة, al-Mughni