A lot of small children recoil at the prospect of putting their hand into a "scary bucket of goo". Obviously they don't understand exactly why they're doing it so they don't want to.
Under 3 months, babies do a lot of sleeping and are quite easy to do. Between 6 months and 3 years old, they are very aware and can be less than willing.
We've found 4 techniques work best.
1. Have an older sibling go first. Mix up a small test batch and have the older one put their hand in. Take some of the alginate and put it on the hand of the younger one while you're doing this. Not only will they see that alginate is actually kinda cool, but most kids would follow an older brother or sister anywhere. This also works, to a lesser degree, with one or both parents going first.
2. Asleep works best, especially for younger children and babies. We recommend mixing our BabyGel (380) alginate with 95°F water so by the time the baby's hand is in, the alginate is the same temperature as the skin on their hands. Babies who are asleep just do not wake up during the procedure because they never realize its happening. Asleep works, but "sleepy" seems to work against us in this context as they can get very grumpy and stubborn when they're sleepy.
3. Snacks work great here- as they do most of the time. Pick your child's favorite snack and offer it to them just as you're getting started with the hand casting. Anytime we can divert some of their attention away from the alginate, we have a better shot at being successful.
4. Calm music or video or just whistling can distract some children from just about anything. It can be used in conjunction with any of the above.
There are days when absolutely nothing works. Best to knock off for a couple of days and start fresh later.