You can open a cash Junior ISA or a stocks and shares Junior ISA, or set up one account of each kind for the same child. It's up to you how you choose to split your investment over these two accounts, but remember that the total payments that can be made into the accounts cannot exceed the current tax year limit of £4,128.
For example, you could invest £2,064 in a stocks and shares Junior ISA during this tax year, as well as £2,064 in a cash Junior ISA, as long as the overall investment for the 2017/18 tax year does not exceed the £4,128 limit.
Alternatively you could invest £3,000 in a stocks and shares Junior ISA and £1,128 in cash, or place the entire £4,128 allowance in stocks and shares. Equally, you could invest more in a cash Junior ISA.
Remember that the £4,128 Junior ISA allowance is a maximum yearly investment amount for the current tax year - you can invest significantly less than this if you wish to, either as a lump sum, or as a regular investment. Please bear in mind that any unused allowance at the end of the tax year is lost and cannot be added to the allowance for subsequent years.