Hi there, Witchy mama!
My imbolc altar will contain bulbs, different flowerbulbs for spring flowers. Ofcourse there will be home made lambs and sheep on it. My altar is more a seasonal table, where a big piece of a elder stem is centerpiece, and where, every season and festival I create a totally new look. I let my son help with this, every time he can be of more help and put in more of his creativity (he is 2.5 now). Of course I light candles with the altar. A Brigid on your altar is a good thing, but personally I try to keep it natural, with natural materials. This is also to keep it close to my son's perspectives, I find that more important now than my own. I have a personal altar upstairs, with statue and symbols, but I only put that out when I use it. It is for night work, not for a day to day basis.
Next to our front door we have a statue that represents the presence of the Goddess as a protector of home and family, and I light a candle with her every evening.
But most important I find what is going on outside, and I will take my son to see that: visit a farm with newly born lambs!
And on a daily basis show him how days lenghten, what grows already in nature, and how animals start to fix their nests (the magpies already started now and so do the doves). There is no altar like the Mother's own lap, is there? :)

Keeping things natural is a good idea, especially when teaching little ones. My oldest is 7 and she is just now reading about the Gods and Goddesses of different pantheons.

Thank you for reminding me about flowers! Last year I had the Witchlets plant a seed in a cup of dirt and they watered it and took care of it, excited to watch it grow.