I think it's ok for kids to be messy cos it's fun for them, but there still needs to be boundries, for example I let Maria pull any toy she wants during the day, but I will always make her tidy up with me, I let her jump in muddy puddles but only if she is wearing her wellies, I let her play with paint and playdoughs as long as she doesn't throw stuff around and stays on the messy mat whilst playing.

Kids need to have fun but still need to be shown responsibility for clearing up their own mess and needs to learn that there are rules.

With digging in the garden, you could give your kids their own spot, so that they can do what they want in it, aslong as it's not throwing mud and stones around cos that to me would come under naughty, you might see differently tho. That is what I'm planning to do for Maria when I get the garden sorted this year, that way the kids will have their own little fun spot to dig holes in, they may even want to plant flowers in their own little bit, it will be fun for them and you wont have to worry about the rest of the garden dug up.