Classifying this signal in dimension and number of channels
I don't feel like it's a very well-posed question, as multi-channel signals are sometimes represented as vector-valued, which would make them multidimensional. However, I assume the answer that is being fished for is:

This is a one-dimensional, two-channel setup. The two channels are the child's height and weight. Each signal is one-dimensional, measured across time. The sample rate of each signal is $1\text{ month}^{-1}$. You could state them mathematically as:

$$
h[n] \equiv \text{ height of child at month } n
$$
$$
w[n] \equiv \text{ weight of child at month } n
$$