For privacy reasons, Electrum will only use the related output to the child transaction so it wont link your other UTXO with it.

The option you have is the "manual method" via coin control linked above.
The disadvantage is you'll have to manually compute the total fee rate of the child and parent transactions by yourself.

Computation is basic math though:
Before that, get the parent transaction's size and absolute fee (electrum displays the size in bytes but it's actually already in vbytes, 'fee' is the total fee, not the 'fee rate')
For example: '250vbytes' and '0.00002500BTC' (2500satoshi)

• Firstly, Create the child transaction as instructed in the link but take note of the size in the 'New Transaction' window but do not finish/send it yet.
• Secondly, to get your target fee rate, get the sum of the parent and child's sizes, for example: '250vbytes' + '150vbytes' = '400vbytes'
• Thirdly, with your target total transaction fee, '50sat/vbytes' for example; multiply it with that total bytes to get the required absolute fee: 400' x 50 = '20000 satoshi'
  Then, Deduct the parent's absolute fee from the above to see how much you need to add to reach that target, for example: 20000sat - 2500sat = '17500satoshi'
• Finally, set that absolute fee or a few sats more as you finish creating the child transaction, tick "Edit fees manually" in the setting icon (upper-right) to be able to set an absolute fee.

Be careful on that though, since the transaction that you're trying to bump with CPFP may also have an unconfirmed parent (or more) with even lower fee rate that'll make your child transaction useless.
That's because your child transaction has to pay for those too to be able to bump the transaction that you're trying to bump.
So check if it's input(s) are all confirmed or has higher fee rate than itself.