Our intimacy has become less frequent since having children. Is that normal?

Yes, it is common after children arrive, but it still needs looking after, not neglecting. Exhaustion, lack of sleep and hormonal changes after giving birth do lower desire for a while. Discuss this openly and with an open heart, without blaming anyone. Plan time for the two of you, even if brief, and share childcare so that neither of you is too tired. Keep up your hygiene and appearance for each other as before. If the situation drags on or comes with pain, talk to a doctor because there may be a treatable health cause. See etiquette of intimacy.