Do they need an anti-slip sole?
'Antiscivolo' — 'an-ti-SHI-vo-lo'. Six syllables. 'Sci' = 'shi'. Stress on third syllable.
Asking about grip on leather-soled shoes — important for Italian cobblestones. A rubber anti-slip pad can be added by a cobbler to the bottom of leather soles.
'Suola antiscivolo' = anti-slip sole. 'Hanno bisogno di' = they need (literally 'have need of'). This is a practical safety question — Italian cobblestones can be very slippery, especially when wet.
Un calzolaio può aggiungere la gomma antiscivolo?
Can a cobbler add anti-slip rubber?
Asking if the modification is possible
La suola è scivolosa?
Is the sole slippery?
Directly asking about grip quality
Ha grip sufficiente per il pavé?
Does it have sufficient grip for cobblestones?
Specific to cobblestone navigation
Venice's 'calli' (narrow streets) and 'campielli' (small squares) become treacherous when wet. Anti-slip rubber pads ('tasselli in gomma') applied to leather soles are a Venetian survival necessity and any local cobbler applies them routinely.