separate trustee and guardian roles.
Likely you parents will take the custody of your children, but they do not control the money. You can set up a defined amount (adjusted by inflation each year) that will be sent to the guardian each year for food and shelter. In case of tuition and medicla bills, the trustee only releases money based on documentation. The trustee should be instructed to buy appropriate insurance for the kids. Sometimes, trustee need do a little research by calling the school and medical billing offices.
Also very recommended is to set the trust transfer date when your youngest children reaches 30 year old. 18 year old kid makes dumb decisions and should not be trusted for the money.