Do you think the toad will laugh at Winnie and think she is a coward? Why is she concerned about its reaction?

This question may sound funny at first, but recall that Winnie is an only child and very lonely, and in Chapter 3 we see that she speaks to the toad and confides in him. She even imagines that he might be listening to her and reacting to her. By promising him that she'll do "something interesting—something that's all [hers,]" she feels a sort of obligation to prove herself to that toad.

As readers, we can sympathize with Winnie's loneliness, boredom, and longing for adventure like in her storybooks. It's understandable that she's thinking of running away from home.

Now let's skip over to Chapter 5, when Winnie wakes up and realizes she's too scared to actually run away. Her "imagination supplied the horrors" of what might happen to her while she's out in the real world by herself. She decides not to leave. Here are Winnie's thoughts in that moment:

"Still, it was galling, this having to admit she was afraid. And when she remembered the toad, she felt even more disheartened. What if the toad should be out by the fence again today? What if he should laugh at her secretly and think she was a coward?"

So, do we readers think the toad will laugh at her and think she's a coward?

Realistically, no. The toad probably doesn't care at all. He's a toad. He probably wasn't paying attention to Winnie during their "conversation" and can't even understand English.

But if you've got a vivid imagination like Winnie does, then maybe so. Maybe the toad heard Winnie's resolve and felt her determination, and now when he learns about her failure to follow through, he'll shun her and think she's nothing but a scared little girl.

Why is Winnie concerned about the toad's reaction?

The toad is all she's got. If she had friends, or siblings, she'd naturally worry about what they'd say when she failed to do the risky thing she said she would do. So instead, Winnie worries about what the toad will think. In this way, the toad is simply an extension of herself, of her own mind, and so you might say that she fears her own self-reproof.

Or, you might say that by making the promise to the toad, she made it more real than if she'd only thought of it in her own mind. Breaking that promise is a big deal, then, so she worries about the consequences.