Remember that all those sweets that your kids get halloween does not make them sweeter. In fact it might be fair to say that you should be expecting more meltdowns that ever.
Children from hard places do not do well when they get out of their routine. Halloween is anything but routine. There is usually changes in schedules at school. Extra snacks etc. Then stores are full of all of the scary masks and costumes. TV and even commercials are showing scary movies and advertisements. These things are scary to anyone but especially kids that have already been traumatized in their early lives.
New sights and new sounds make our children very uncomfortable. Then taking them out to parties or trick of treating is often to over stimulating for them. Extra sugar is no help.
We want our children to enjoy and experience things that others kids experience. However we have to present it in a different way. Make sure your kids know what is happening and what to expect. Avoid scary costumes and no haunted houses or scary movies. If going trick or treating or to a party make sure they have ate a healthy meal first to help prevent gorging on candy and sugar. Lots of water to keep them hydrated can help off set tantrums.
Go early and get home early so they have time to wind down and relax before bed time with a bed time story or a funny movie.