Children should wear ইহরাম garments with leniency applied — the parent makes niyyah for young children, and no fidyah is required if children violate ইহরাম restrictions.
Children can and should wear ইহরাম garments when performing হজ্জ or উমরাহ, 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 ইহরাম garments sized for children are readily available in shops near the Haram. Girls wear their normal modest clothing, as women's ইহরাম 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 ইহরাম restrictions: if a child covers their head (boys), uses scented products, or otherwise violates ইহরাম restrictions, no fidyah (penalty) is required জমহুর উলামাদের মতে. 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 ইহরাম etiquette to the extent their age allows.
Source: Sunan Abu Dawud; al-Nawawi, al-Majmu'; Ibn Qudamah, al-Mughni