Do you think it was wise of Mr. Beaver to take the time to pack supplies? Why or why not?

Actually, it was Mrs. Beaver, who took the time to pick up items for their journey. Everyone else was in such a hurry to be off that they didn't think about the things they might need. Having taken care of a family.... Mrs. Beaver knew that supplies would likely be needed.

“There’s no time to lose,” everyone began bundling themselves into coats, except Mrs Beaver, who started picking up sacks
and laying them on the table and said: “Now, Mr Beaver, just reach down that ham. And here’s a packet of tea, and there’s sugar, and some matches. And if someone will get two or three loaves out of the crock over there in the corner.”
“What are you doing, Mrs Beaver?” exclaimed Susan.
“Packing a load for each of us, dearie,” said Mrs Beaver very coolly.
“You didn’t think we’d set out on a journey with nothing to eat, did you?”

After spending the night in the cave, we can see that Mrs. Beaver's supplies were greatly appreciated by all....

So down the steep bank they went and back to the cave, and Mr Beaver cut some of the bread and ham into sandwiches and Mrs Beaver poured out the tea and everyone enjoyed themselves. But long before they had finished enjoying themselves Mr Beaver said, “Time to be moving on now.”