Move Between Input Cells Only

I have designed a form for my wife to use in school to help teach fractions and decimals.

The children will enter the numerator into a cell (say D2) and the denominator into another cell (say D6) and the decimal equivalent is then displayed to 2 decimal places in a third merged cell (say C8 - E8).

What I would like to do is this:

When the worksheet opens, the only cell that I want the children to be able to enter data into is the first cell (ie D2). When they enter a value (say 1 for the numerator) I'd like the return key to move the data entry to the next cell (which is D6). After this, when they have entered another value (say 3 - they are invesigating thirds for example), I'd like the return key to move to the next numerator cell (which is H2), then denominator cell (H6) and so on.

How do I do this? I have never created forms or macros and I suspect that this is how to do this.

For the first set of fractions that they are investigating, thirds, I'd like to limit their data entry in the denominator cells only (D6, H6, L6, P6, etc) to 3 and reject any other number. Later in the worksheet, they will be investigating quarters, fifths and so on.

How do I do this? I don't want then to use any list or drop-down, simply to type a value into the cell and it be accepted (if it is a 3 when investigating thirds) if it is correct or simply ignored if it isn't. Is this possible?

way, especially if it involves anything complicated like macros. Being active on lots of forums it is quite depressing when I see that someone has answered a poster's question but the answer seems to either assume a high level of knowledge or tends to assume that certain steps are taken without actually stating them.