Will the rent cover the mortgage, after tax, management fees and periods of non rental?
If your child is very young, I would stay where you are and overpay the mortgage as much as you can and keep the tracker. Long term this will save you money.
Will you be able to fit 2 children into the apartment for a few years and maybe plan for a move in 5+ years when you are financially in a much better place?
Also, we are in similar position, and the bank wanted us to have 2k + 250 per child at minimum, before making a mortgage payment. Finally, think about the variable rate on the second mortgage in the region of 4-5%.
If you wait, you maximise your financial health in a few years time.