hey Sunshine good luck with this and try not worry, as they may pick up on your stress.
Any suggestions on how to encourage them to slow down?
Google mindful eating and mindful eating for children.
This is fab, it's the basics healthy-kids.com.au/parents/developing-positive-eating-behaviours/mindful-eating/
I've done a few things like mindfully eating a raisin or mindfully eating a Maltesers. This is as an adult with eating disorder.
mindful eating is fun and can be presented a game. You take your small food item, feel it, smell it, describe it (limited language so nice, fruity, chocolatey etc) put it on your tongue, taste it, let Maltesers dissolved, chew raisin etc etc. We also did a thing at church where kids got to eat chocolate buttons by letting them dissolve on their tongue, we timed the kids. It was also fun.
The trick (as I am someone who eats too much) is to enjoy what you have now without wanting to move instantly on to the next thing. I say trick because as time goes and kids eat slower they will taste the food more, feel fuller and know when full and be able to stop.
I never encourage my kids to finish up their food, I just encourage some healthy eating like some veg with dinner.
I do not belittle choices, this is hard as we are trained to belittle kids choices for some things, and really encourage healthy eating and it is a fine balance because too much emphasise on healthy eating might mean they go the other way.
The top pick when I googled 'Mindful eating for kids' was Summer Family 2013 patient care handout - The Center for Mindful eating..... worth a look.
Take a look, there is stuff about curiosity making kids interested in every aspect of eating. So you could develop some ideas around that. If they can resist the uncooked cake mix/batter for long enough to bake into cakes they can a) decorate them b) eat them - in limited amounts of course c) give them as gifts to grandma or next door neighbours etc
Work up to it, small amounts, healthy food, maybe fruit pie or fruit flan is healthier than cakes, i am not sure, do small amounts and lots of praise when they are able to resist food or wait. Maybe grandma could send a special card or gift in return for the cakes, showing the kids that eating less themselves means more to share!
This is all amateur ideas from a person with an eating disorder. Luckily my kids (nothing to do with me) eat very healthily!
Good luck.