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When they get a fever, they can sometimes have a seizure.
Literal
Fever [subject-が] come-out [when-と] seizure [object-を] cause [thing-that-happens-ことがあります].
A medical advisory. 熱が出る ('a fever comes out') is the standard collocation for 'running a fever' in Japanese — the fever is framed as something that 'emerges' rather than something one 'has.' ひきつけ (or 引き付け) is the common word for 'febrile seizure,' 'convulsion in a small child triggered by high fever' — a specific medical event that parents are warned about. ~を起こす is the set verb for 'cause/trigger (a symptom/event)': ひきつけを起こす = 'have a seizure.' The frame ~ことがあります ('there are times when X') marks the occurrence as possible but not certain.