The uncle holding my children's share of the estate is using it for himself. What can I do?

An orphan's property is a very heavy trust, and its misuse can be taken to court. Allah says: “Allah doth command you to render back your Trusts to those to whom they are due” (Surah an-Nisa' 4:58). Start by asking for a written statement: how much was received, where it is kept, and what has been spent on the children. Ask politely but clearly. If he refuses, or his explanation is unreasonable, keep all the evidence and get advice from the Syariah Court, which can review the management of that property and change the guardian if necessary. Meanwhile, do not let this dispute cut your children off from their late father's family. See guardians of an orphan's property.