It is unusual for children under 7 to get hayfever. It does sometimes appear in toddlers (I don't know how old your child is?) and some pharmacists do sell remedies that they consider safe for toddlers, but there doesn't seem to be anything for very young babies. Babies can have symptoms that look very like hay fever but often are something else, such as a cold. Even if your baby does have hay fever or another allergy, there aren't really any effective medications for children under a year old.There has been a conversation about this on Mumsnet which you may find helpful.
If your child does have hay fever and is too young for medical treatment, it can help to wash his or her face and hands when you come in from outdoors and change outer clothes. You can find some more advice here but really you should see a doctor if you are worried.