It’s not at all an uncommon thing for kids to be apprehensive about messy play. Both my kids went through various stages of it. My number one thing is don’t push, it’s likely a phase. Try the activities and play with them yourself as an example , try them when you have a playdate and let the other kids be examples too. Also try to use themes your kids love to add a familiar and comfortable element to the new and not so comfortable material.