We start by simplifying the given function $f(x)$:
\begin{align*}
f(x) &= 2\sin^2x + 2\sin x\cos x \\
&= \sin 2x + 1 - \cos 2x \\
&= \sqrt{2}\sin \left(2x - \frac{\pi}{4}\right) + 1.
\end{align*}

The smallest positive period $T$ can be determined by looking at the argument of the sine function. Since the $\sin$ function has a period of $2\pi$, and the argument of $\sin$ here is $2x - \frac{\pi}{4}$, we conclude the smallest positive period for $f(x)$ is:
$$T = \frac{2\pi}{2} = \pi.$$

Next, we consider the interval in which $\sqrt{2}\sin \left(2x - \frac{\pi}{4}\right)$ is monotonic decreasing. For the sine function, this occurs when:
$$\frac{\pi}{2} + 2k\pi \leq 2x - \frac{\pi}{4} \leq \frac{3\pi}{2} + 2k\pi, \quad k \in \mathbb{Z}.$$
By solving these inequalities for $x$, we find:
$$\frac{3\pi}{8} + k\pi \leq x \leq \frac{7\pi}{8} + k\pi, \quad k \in \mathbb{Z}.$$
For $k=0$, we get a monotonic decreasing interval for $f(x)$ to be:
$$\left[\frac{3\pi}{8}, \frac{7\pi}{8}\right].$$

Thus, the smallest positive period of $f(x)$ is $\pi$, and one interval where $f(x)$ is monotonic decreasing is $\left[\frac{3\pi}{8}, \frac{7\pi}{8}\right]$. Therefore, the correct answer is \boxed{\text{B}}.